What a week calculator actually does
A week calculator converts between calendar dates and weeks. At its core it answers three questions: how many weeks lie between two dates, what date is N weeks from now, and what week number is a given date. Everything else — pregnancy tracking, sprint planning, payroll periods, contract deadlines, leave planning, T-bill maturities — is a practical application of those three primitives.
The arithmetic sounds trivial: divide the number of days by 7. But real date counting is full of traps. Months have 28, 29, 30, or 31 days. Leap years add a day every four years (with century exceptions). The ISO week year can differ from the calendar year by up to three days at each end. Inclusive counting adds one day; exclusive counting does not. Business-day counting skips weekends. A good week calculator handles all of these automatically and shows you exactly what it counted — which is what this tool does across six modes with interactive visual confirmation at every step.
The live widget at the top of this page shows you today's ISO week number, a progress bar tracking how far through the current week and year you are, and your local timezone offset. It updates every minute so you always have the current week reference at a glance.
How to use each mode
Weeks Between — the core mode
Enter a start date and an end date. The results update instantly — no Calculate button. Below the hero result you get five secondary stat cards (total days, decimal weeks, months and days, business days, and the ISO week numbers of both endpoints). Below those you get three interactive visualisations:
- SVG timeline — a hoverable, clickable timeline segmented by month, proportional to the number of days in each month inside your range. Hover any segment to see its tooltip with the full date range and day count. Click a segment to see a detailed breakdown of that month's days. A dashed amber "today" marker shows where the current date falls on the timeline.
- Week grid — every week that touches the range as a tile, showing its ISO week number, Monday–Sunday dates, and how many of its seven days fall inside the range. Click any tile to expand a detail panel with all seven dates broken down and highlighted by in-range or out-of-range status.
- Calendar heatmap — one mini month calendar per month in the range, with every in-range day filled in. Hover any day for its full date and ISO week number. Today is ringed in amber. If the range spans more than 12 months, navigation buttons let you browse beyond the first view.
Toggle Inclusive to add one day (so both start and end dates count). Toggle Business to exclude Saturdays and Sundays. The swap button flips the two dates. The Today buttons set either field to the current date. Two export buttons generate a CSV of the full result or an .ics calendar file with the start and end dates as events.
Add / Subtract — date projection
Pick a start date, enter a number of weeks, and choose Add or Subtract. Optionally add extra days. The result is the exact date, its day of the week, its ISO week number, and a mini calendar of the result month with the result date highlighted. Quick chips for 1, 2, 4, 8, 12, 26, and 52 weeks let you jump to the most common projections with one click — useful for 12-week contract terms, 26-week Treasury bills, and 52-week annual planning. Export the result to your calendar as an all-day event.
Countdown — weeks from today, live
Pick a target date, or tap one of the event chips (Christmas, New Year, Halloween, Thanksgiving, Valentine's Day, summer solstice, US Independence Day, UK Boxing Day). The countdown updates every second and shows five tick boxes: weeks, days, hours, minutes, and seconds — each pulses briefly when its value changes. A circular progress ring shows how much of the total span has elapsed. Below the ticker you get total days, total hours, decimal weeks, and the ISO week number of the target date. Switch to Age in weeks to enter a birthdate and see your exact age in weeks and days, your next round-week milestone, and how many weeks until your next birthday. Export the target date as an .ics calendar event.
Week Number — ISO 8601 lookup
Two sub-modes. Date → week number shows the ISO week number, ISO week year, Monday–Sunday range, and day of the week for any date. Week → dates goes the other direction: enter a year and a week number and get the seven dates in that week. Below both is an interactive year overview grid showing all 52 or 53 ISO weeks of a chosen year, with the current week highlighted. Click any week tile to jump into week → dates mode for that week.
Pregnancy — weeks from LMP with baby sizes
Enter the first day of your last menstrual period. The tool calculates your current gestational age in weeks and days, your trimester, your estimated due date (LMP + 280 days), days remaining, and a big colourful baby-size comparison — from a poppy seed at week 4 to a watermelon at week 39 — with approximate length and weight. Below that a progress bar shows how far through the 40 weeks you are, and a milestone list colour-codes past, current, and future milestones at 4, 8, 12, 16, 20, 24, 28, 32, 36, and 40 weeks. Export the due date as an .ics calendar event.
Sprint — agile planner with gantt chart
Enter the Monday your first sprint starts, choose a sprint length of 1–4 weeks, and set how many sprints you want to plan. The tool generates an interactive gantt chart — each week is a column, each sprint is a row of coloured cells, and today's week is outlined in amber. Below the gantt, individual sprint cards show the number, Monday–Sunday dates, ISO week numbers, and working days (5 per week). Export the whole sprint plan as CSV, as .ics calendar events, or as plain text for your ticket tracker.
The mathematics of counting weeks
All six modes reduce to a handful of arithmetic operations on dates. Here is exactly what the tool computes.
The one subtlety is that "difference in days" must be computed on calendar dates, not on timestamps. If you subtract two Date objects that carry a time-of-day, a daylight-saving transition can turn a 7-day span into 6.958 days and round to 6. This calculator normalises both dates to midnight before subtracting, then rounds to the nearest whole day, so DST never causes an off-by-one error.
How many weeks are in a year?
A standard year has 52 weeks and 1 day. A leap year has 52 weeks and 2 days. But the number of numbered weeks in a year — the ISO week count — is either 52 or 53, and that is a different question.
| Year type | Days | Exact weeks | ISO weeks |
|---|---|---|---|
| Common year | 365 | 52 weeks + 1 day | 52 or 53 |
| Leap year | 366 | 52 weeks + 2 days | 52 or 53 |
| Average Gregorian year | 365.2425 | 52.1775 weeks | — |
| Decade | 3,652.4 | 521.77 weeks | — |
| Century (Gregorian) | 36,524.25 | 5,217.75 weeks | — |
Which years have 53 ISO weeks? A year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday. Recent examples: 2015, 2020, 2026, 2032, 2037. The pattern repeats on a 28-year cycle (with century interruptions).
How many weeks are in a month?
Months are not divisible by 7, so no month contains an exact whole number of weeks except February in a common year (exactly 4 weeks). Here is every month:
| Month | Days (common) | Days (leap) | Weeks + days (common) |
|---|---|---|---|
| January | 31 | 31 | 4 weeks + 3 days |
| February | 28 | 29 | 4 weeks + 0 days |
| March | 31 | 31 | 4 weeks + 3 days |
| April | 30 | 30 | 4 weeks + 2 days |
| May | 31 | 31 | 4 weeks + 3 days |
| June | 30 | 30 | 4 weeks + 2 days |
| July | 31 | 31 | 4 weeks + 3 days |
| August | 31 | 31 | 4 weeks + 3 days |
| September | 30 | 30 | 4 weeks + 2 days |
| October | 31 | 31 | 4 weeks + 3 days |
| November | 30 | 30 | 4 weeks + 2 days |
| December | 31 | 31 | 4 weeks + 3 days |
On average a month is 4.348 weeks (52.1775 ÷ 12). That is why "4 weeks" is a common budgeting approximation but never exactly right for a calendar month. Use the Weeks Between mode with a month as both endpoints to see the exact figure for any specific month.
ISO 8601 week numbering explained
ISO 8601 is the international standard that defines how weeks are numbered. Three rules define it:
- Weeks run Monday to Sunday. Monday is day 1 and Sunday is day 7. This is the standard across most of Europe, Asia, and Oceania. The US, Canada, and parts of Latin America often number weeks starting on Sunday — this tool follows ISO.
- Week 1 is the week containing 4 January. Equivalently, it is the week containing the first Thursday of the year. This guarantees that every year has either 52 or 53 weeks and that week 1 always contains at least four days of the new year.
- The ISO week year is the year of that week's Thursday. This is why 29 December 2025 (a Monday) belongs to ISO week 1 of 2026, not week 52 of 2025. The ISO year can differ from the calendar year by up to three days at each end.
ISO week numbers are used in payroll, logistics, manufacturing planning, retail merchandising, school timetables, and international project management. If a contract says "delivery in week 42", ISO 8601 tells both parties exactly which seven days are meant — something calendar dates cannot do without a year.
This calculator's Week Number mode implements the full ISO 8601 algorithm, including the year-boundary edge cases. The year overview grid shows every week of any year with its Monday and Sunday dates, so you can verify any lookup visually.
Inclusive vs exclusive counting — and why it matters
The single most common source of week-counting errors is forgetting to ask whether the end date counts. Consider a hotel stay from 1 June to 8 June:
- Exclusive (the gap between the dates): 7 days — exactly 1 week. This is the correct count for nights stayed, because you check out on the morning of the 8th and do not sleep there that night.
- Inclusive (both dates counted): 8 days — 1 week and 1 day. This is the correct count for days of leave if the policy says "leave through the 8th".
Neither is wrong — they answer different questions. The rule of thumb: use exclusive when counting elapsed time between two moments ("weeks until the deadline"), and inclusive when counting whole days that both start and end on a counted day ("days of an event"). This calculator gives you a one-click toggle so you can see both numbers side by side.
Calendar weeks vs business weeks
A calendar week is seven consecutive days. A business week (or work week) is the subset of those days that are not weekends — five days in most countries, six in some. A four-week calendar span contains 28 days but only 20 working days in a Monday–Friday economy.
The Business toggle in the Weeks Between mode excludes Saturdays and Sundays from the day count before dividing. A span that is 4 weeks and 3 calendar days becomes 4 work weeks and 3 working days — which is exactly what you need for project effort estimates, timesheet reconciliation, and SLA calculations. Public holidays are not automatically removed because they vary by country and region; subtract them manually if your contract requires it.
Pregnancy weeks explained
Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception. This is because the exact conception date is rarely known, while the LMP date usually is. A full-term pregnancy is 40 weeks (280 days) from the LMP, which corresponds to about 38 weeks from conception.
Naegele's rule estimates the due date: add 7 days to the LMP and subtract 3 months (equivalently, add 280 days). For a 28-day cycle this is accurate to within a few days for most pregnancies; for irregular cycles, ultrasound dating in the first trimester is more reliable.
| Trimester | Weeks | What is happening |
|---|---|---|
| First | 0w0d – 13w6d | Organ formation, highest miscarriage risk, morning sickness common |
| Second | 14w0d – 27w6d | Movement felt, anatomy scan around 20 weeks, generally the most comfortable trimester |
| Third | 28w0d – birth | Rapid growth, viability from ~24 weeks with intensive care, preparation for birth |
The Pregnancy mode shows your current week and day, trimester, due date, days remaining, and the dates of every standard milestone. A baby-size comparison shows how big the fetus is at your current week — from a poppy seed at week 4, to a blueberry at week 7, an avocado at week 16, a banana at week 20, an eggplant at week 28, a pineapple at week 33, and a watermelon at week 39. The milestone list is colour-coded: past milestones are green, the current milestone is amber, future milestones are grey.
⚠ This tool provides educational estimates only. It is not a medical device and does not replace clinical assessment. Always follow your healthcare provider's guidance.
Sprint planning with weeks
Agile teams plan in sprints — fixed-length development cycles, usually 1, 2, 3, or 4 weeks. Two-week sprints are the most common because they balance planning overhead against feedback frequency. A typical quarterly plan contains six 2-week sprints.
The Sprint mode generates both a gantt chart (visual) and individual sprint cards (detailed). Each card shows the sprint number, Monday–Sunday dates, ISO week numbers, and working days. The gantt visualises the entire plan as a grid of weeks × sprints with today's week highlighted, so you can see at a glance where each sprint sits relative to the others.
Two practical tips from teams that plan well: anchor sprints to Mondays so that every sprint starts on ISO week 1 day, and leave one buffer week per quarter for holidays, incidents, and carry-over. A six-sprint quarter with 2-week sprints covers 12 weeks; the remaining 1 week in a 13-week quarter is your buffer.
Payroll and work weeks
Payroll periods are defined by the employer, not by the calendar, and they come in several standard shapes:
| Pay frequency | Periods per year | Weeks per period |
|---|---|---|
| Weekly | 52 | 1.000 |
| Bi-weekly (every 2 weeks) | 26 | 2.000 |
| Semi-monthly (1st & 15th) | 24 | ~2.167 |
| Monthly | 12 | ~4.333 |
Note that bi-weekly (26 periods of exactly 14 days) and semi-monthly (24 periods of 15 or 16 days) are different. A year with 26 bi-weekly pay periods contains 52 weeks; 24 semi-monthly periods contain 24 variable-length spans that add up to 365 days. To count pay periods between two dates, use the Weeks Between mode with business days and divide by 2 for bi-weekly.
Tax years, fiscal years, and financial weeks
Tax years vary widely by country, and week counting matters for fiscal planning and filing deadlines.
| Country | Tax year | Weeks (approx) |
|---|---|---|
| United States (federal) | 1 Jan – 31 Dec | 52 weeks + 1–2 days |
| United Kingdom | 6 Apr – 5 Apr | 52 weeks + 2 days |
| Australia | 1 Jul – 30 Jun | 52 weeks + 1–2 days |
| India | 1 Apr – 31 Mar | 52 weeks + 1–2 days |
| Japan | 1 Apr – 31 Mar | 52 weeks + 1–2 days |
| Germany | 1 Jan – 31 Dec | 52 weeks + 1–2 days |
Financial weeks are also used in retail merchandising (the NRF 4-5-4 calendar), US Treasury bill maturities (4, 8, 13, 17, 26, 52 weeks), and bond coupon dates. The Weeks Between mode handles all of these — pick the start and end dates of the fiscal period and it tells you the exact number of weeks.
School terms and academic weeks
Education systems count in weeks because timetables repeat weekly. Typical figures:
- UK state schools: 38–39 teaching weeks per year, split across three terms (autumn ~14 weeks, spring ~12 weeks, summer ~13 weeks).
- US K-12: 36 weeks of instruction (180 school days ÷ 5 days per week), typically split across two semesters of 18 weeks.
- US universities: 15–16 week semesters, plus a 1-week exam period. A full academic year is 30–32 teaching weeks.
- Australian schools: 40 teaching weeks across four terms of roughly 10 weeks each.
The Weeks Between mode is ideal for verifying term lengths, calculating how many teaching weeks remain before an exam, or planning a study schedule across a semester break.
Contracts, notices, and statutory periods
Many legal and business processes are expressed in weeks. Here are common examples and how to calculate them:
| Period | Weeks | Days |
|---|---|---|
| 2-week notice period | 2 | 14 |
| 30-day return window | 4 + 2 days | 30 |
| 6-week probation | 6 | 42 |
| 12-week contract term | 12 | 84 |
| 13-week quarter (T-bill) | 13 | 91 |
| 26-week T-bill | 26 | 182 |
| 40-week pregnancy | 40 | 280 |
| 52-week annual savings | 52 | 364 |
| 90-day statutory warranty | 12 + 6 days | 90 |
When a contract says "within 12 weeks from signing", use the Add/Subtract mode. Enter the signing date, choose Add, enter 12 weeks, and the tool computes the exact deadline and its ISO week number. When a contract says "90 days from today", use the Countdown mode and enter the date 90 days from today.
Worked examples
How many weeks between 1 January 2026 and 1 July 2026?
1 January to 1 July 2026 is 181 days (2026 is not a leap year; January 31 + February 28 + March 31 + April 30 + May 31 + June 30 = 181). 181 ÷ 7 = 25 weeks and 6 days. In decimal terms that is 25.857 weeks. Inclusive counting would give 182 days = 26 weeks exactly.
What date is 12 weeks from 15 March 2026?
12 × 7 = 84 days. 15 March + 84 days = 7 June 2026, a Sunday. Its ISO week number is 23 of 2026. If your contract runs "12 weeks from signing", the deadline is 7 June.
How many weeks until Christmas 2026?
From 11 November 2026 to 25 December 2026 is 44 days = 6 weeks and 2 days (6.286 decimal weeks). The Countdown mode shows this live to the second, plus the total hours and minutes, and can export the date to your calendar as an .ics event.
How many business weeks between 5 January and 30 March 2026?
5 January to 30 March 2026 is 84 calendar days = exactly 12 weeks. Because 12 weeks is 84 days and every 7-day block contains exactly 5 weekdays, the business-day count is 60 working days = 12 work weeks. There are no public holidays in the range for most countries, so the business count matches the calendar count in weeks.
What week number is 29 December 2025?
29 December 2025 is a Monday, and it falls in ISO week 1 of 2026 — not week 52 of 2025. This is the classic ISO year-boundary case. The Week Number mode shows the ISO week year alongside the week number so this never surprises you.
How many weeks pregnant at 98 days from LMP?
98 ÷ 7 = exactly 14 weeks and 0 days — the start of the second trimester. The tool displays the corresponding baby size (lemon, ~8.7 cm) and highlights the 12-week and 16-week milestones as past and future respectively. Due date would be 182 days from today.
Common week-counting mistakes
- Forgetting inclusive vs exclusive. A 7-day hotel stay from Monday to the following Monday is 7 nights (exclusive) but 8 calendar days (inclusive). Get this wrong and you over-book by a day.
- Confusing weeks and week numbers. "Week 12" and "12 weeks" are different things. Week 12 is the twelfth numbered week of a year; 12 weeks is a duration of 84 days.
- Assuming every year has 52 weeks. A year has 52 weeks plus 1 or 2 days. In ISO terms it has 52 or 53 numbered weeks. Budgeting 52 weeks × 5 days = 260 working days is right on average but wrong in any specific year with 53 weeks.
- Counting months as 4 weeks. A month is 4.35 weeks on average. Treating every month as exactly 4 weeks drifts by 7 days per quarter — a full week of error every 3 months.
- Timezone and DST errors. Subtracting two timestamps across a daylight-saving change can give 6.958 days, which floors to 6. Always normalise to calendar dates first. This tool does that for you.
- Using the calendar year for the ISO week year. 1 January can belong to week 52 or 53 of the previous ISO year; late December can belong to week 1 of the next. Use the ISO week year, not the calendar year.
- Assuming business days exclude holidays. The Business toggle excludes weekends only. Public holidays vary by country, state, and employer, so they must be subtracted manually.
- Counting pregnancy from conception. Clinical gestational age is counted from the LMP, which is about 2 weeks before conception. A "6-week pregnant" embryo is about 4 weeks post-conception.
Week number quick reference
| Duration | Weeks + days | Decimal weeks | Working days (Mon–Fri) |
|---|---|---|---|
| 1 month (average) | 4 weeks + 2–3 days | 4.348 | ~22 |
| 1 quarter | 13 weeks + 0–1 day | 13.0–13.14 | 65 |
| Half year | 26 weeks + 0–2 days | 26.07–26.29 | 130 |
| 1 year (common) | 52 weeks + 1 day | 52.143 | 260 |
| 1 year (leap) | 52 weeks + 2 days | 52.286 | 261 |
| 1 decade | 521 weeks + 5 days | 521.77 | 2,609 |
| 1 century | 5,217 weeks + 5 days | 5,217.75 | 26,089 |
| 40 weeks (pregnancy) | 40 weeks + 0 days | 40.0 | 200 |
| 90 days | 12 weeks + 6 days | 12.857 | 64 |
| 100 days | 14 weeks + 2 days | 14.286 | 71 |
| 180 days | 25 weeks + 5 days | 25.714 | 129 |
| 365 days | 52 weeks + 1 day | 52.143 | 261 |
A short history of the seven-day week
The seven-day week is one of the oldest continuously used units of time. It appears in Mesopotamian astronomy around 600 BCE, where the seven visible celestial bodies — Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn — gave the days their names. The Roman world adopted a seven-day cycle in the 1st century CE, and the names of the days in English still carry the old Germanic and Roman gods: Sun, Moon, Tiw (Mars), Woden (Mercury), Thor (Jupiter), Frigg (Venus), Saturn.
The seven-day week is not astronomically derived. A month approximates the Moon's orbit and a year approximates the Sun's, but nothing in the sky cycles every seven days. The week is a social convention so useful that it has survived every calendar reform. The French Revolutionary calendar tried a ten-day décade in 1793 and abandoned it within twelve years. The Soviet Union tried a five-day and then a six-day week in the 1920s and 1930s and reverted to seven in 1940.
ISO 8601, which standardised week numbering internationally, was first published in 1988 and is now maintained by the International Organization for Standardization and the International Electrotechnical Commission. It defines the Monday-start week, the 4-January rule for week 1, and the ISO week year. Its purpose is to make "week 34" unambiguous in international trade — without it, a shipment scheduled for "week 34" could leave a factory on five different dates depending on which country you asked.
Limitations
- All calculations use the proleptic Gregorian calendar. Dates before 1582 (when the Gregorian calendar was introduced) use the same arithmetic but do not reflect the Julian calendar that was in use at the time.
- The Business toggle excludes Saturdays and Sundays only. Public holidays, company shutdowns, and regional non-working days are not automatically removed.
- Pregnancy estimates use Naegele's rule from the LMP. They do not account for irregular menstrual cycles, IVF transfer dates, or ultrasound dating.
- Baby-size comparisons are approximate fruit/vegetable comparisons used in common pregnancy apps. Sizes vary widely between individual fetuses at the same gestational age.
- The sprint planner assumes sprints start on a Monday and run for whole weeks. Mid-week sprint starts can be modelled by choosing the Monday of the week you want to start.
- ISO week numbers follow ISO 8601 only. The US Sunday-start convention and the Excel WEEKNUM function's various return types are not implemented.
- The heatmap navigation shows 12 months at a time for ranges that span more than a year. Longer ranges can be navigated month by month.
- ICS calendar export creates all-day events. Timezone-aware timed events are not supported.
Frequently asked questions
How do I calculate weeks between two dates?
Subtract the earlier date from the later date to get total days, then divide by 7. The whole-number result is full weeks and the remainder is leftover days. Example: 100 days = 14 weeks and 2 days. This calculator does it automatically, handles leap years, and shows total days, decimal weeks, months, business days, and ISO week numbers at once.
How many weeks are in a year?
A standard year has 52 weeks and 1 day (365 ÷ 7 = 52.143). A leap year has 52 weeks and 2 days (366 ÷ 7 = 52.286). In the ISO 8601 week system, a year has 52 or 53 numbered weeks — 2026 has 53 ISO weeks.
How many weeks are in a month?
On average about 4.35 weeks. February in a non-leap year is exactly 4 weeks (28 days). A 31-day month is 4 weeks and 3 days. The number of complete calendar weeks inside a month is always 4 or 5.
Does the week calculator include leap years?
Yes. Dates are compared by their actual calendar day counts, so 29 February is automatically included whenever it falls inside the range. No manual adjustment is needed.
What is the difference between inclusive and exclusive counting?
Exclusive counting measures the gap between dates — it does not count the start date. Inclusive counting adds one day so both the start and end dates count. Example: 1 January to 8 January is 7 days exclusive (exactly 1 week) but 8 days inclusive (1 week and 1 day). Use inclusive when a rule says 'through the end date'.
What is an ISO week number?
ISO 8601 is the international standard for week numbering. Weeks run Monday to Sunday, and week 1 is the week that contains 4 January (equivalently, the first Thursday of the year). Week numbers range from 1 to 53. Most of Europe, Asia, and Oceania use ISO numbering.
Why does the ISO week year sometimes differ from the calendar year?
Because the ISO week belongs to whichever year holds its Thursday. 29 December 2025 is a Monday that belongs to ISO week 1 of 2026. Similarly, 1 January can fall in week 52 or 53 of the previous ISO year. The ISO year label can therefore differ from the calendar year by up to three days at each end.
How many weeks are in a leap year?
52 weeks and 2 days (366 ÷ 7 = 52.286). In ISO terms a leap year has either 52 or 53 numbered weeks depending on which weekday 1 January falls on.
How do I calculate business weeks?
Switch the day-counting mode to Business. The calculator excludes Saturdays and Sundays from the total day count, then divides by 5 to give full work weeks. A 10-working-day span is exactly 2 work weeks.
How many weeks is a pregnancy?
A full-term pregnancy is dated as 40 weeks (280 days) from the first day of the last menstrual period. Post-term is generally considered 42 weeks or more. The Pregnancy mode shows your current week and day, trimester, due date, and standard milestone dates.
How do I calculate my pregnancy week from my LMP?
Count the days from the first day of your last menstrual period to today, then divide by 7. For example, 98 days = 14 weeks and 0 days. Due date uses Naegele's rule: LMP + 280 days (or LMP + 7 days − 3 months).
What are the trimesters in weeks?
First trimester: weeks 0 to 13 and 6 days. Second trimester: weeks 14 and 0 days to 27 and 6 days. Third trimester: week 28 and 0 days to birth. The calculator displays the trimester based on your current gestational age.
How do I add 12 weeks to a date?
Add 84 days (12 × 7) to the start date. The Add/Subtract mode does this instantly and also shows the resulting ISO week number, day of the week, and a mini calendar. For example, 1 January 2026 + 12 weeks = 26 March 2026.
How do I subtract 6 weeks from a date?
Subtract 42 days (6 × 7). Toggle the operation to Subtract in the Add/Subtract mode. For example, 15 March 2026 − 6 weeks = 1 February 2026.
How many weeks until Christmas?
Use the Countdown mode and tap the Christmas preset. It shows the exact weeks, days, hours, minutes, and seconds until 25 December, updated live, with a one-click .ics calendar export. Christmas 2026 falls on Friday, 25 December.
How many weeks are in a quarter?
A quarter is 13 weeks plus 0 or 1 day, because 91 ÷ 7 = 13 exactly and most quarters have 91 or 92 days. A leap-year quarter that includes 29 February has 13 weeks and 1 day.
How many weeks are in a school year?
Most school years run 36 to 40 weeks of teaching, spread across a 52-week calendar year with holidays. UK school years are typically 38–39 weeks; US K-12 years are typically 36 weeks (180 days at 5 days per week).
How do I count weeks for payroll?
A payroll week is a fixed 7-day workweek defined by the employer. To count pay weeks between two dates, use the weeks-between mode with business days enabled, or divide the number of paydays by the pay frequency (52 for weekly, 26 for bi-weekly, 24 for semi-monthly).
What is a sprint in agile and how many weeks is it?
A sprint is a fixed development cycle in Scrum. Common lengths are 1, 2, 3, or 4 weeks, with 2 weeks being the most common. The Sprint mode lets you set the length and number of sprints and shows each sprint's dates, ISO week numbers, and working days with an interactive gantt chart.
How many weeks are in a US Treasury bill?
Standard T-bill maturities are 4, 8, 13, 17, 26, and 52 weeks. The 13-week and 26-week bills are the most actively traded. Use the Add/Subtract mode to project the maturity date from a purchase date.
How do I calculate my age in weeks?
Count the days since your birthdate and divide by 7. A 30-year-old is roughly 1,565 weeks old. The Countdown mode has an 'Age in weeks' toggle that shows your exact age in weeks and days, your next round-week milestone, and how many weeks until your next birthday.
What is the current week number?
The live widget at the top of this page shows the current ISO week number, the day of the week, a progress bar showing how far through the current week and year you are, and your local timezone. It updates every minute.
Why do some years have 53 weeks?
A year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday. Because 52 weeks is 364 days, the extra 1 or 2 days in a calendar year occasionally tip over into an extra numbered week. 2026 has 53 ISO weeks.
How many weeks are in a decade or century?
A decade is about 521.8 weeks (10 × 52.143) and a century is about 5,217.7 weeks. Over long spans the Gregorian leap-year rule matters, so exact week counts depend on the specific start and end dates.
Does this week calculator work offline?
Yes. All six modes, all visualisations, and all date arithmetic run in your browser with vanilla JavaScript. There are no network requests after the page loads.
Can I count weeks excluding weekends and holidays?
Weekends are excluded with the Business toggle. Public holidays are not automatically excluded because they vary by country and region — subtract them manually if your contract requires it.
What is the difference between a calendar week and a work week?
A calendar week is any 7-day block (usually Monday–Sunday). A work week is the 5 working days inside that block, Monday–Friday in most countries. A 4-week calendar span contains 20 working days but 28 total days.
How do I find the date 90 days from today in weeks?
90 days is 12 weeks and 6 days. Use the Add/Subtract mode: today + 90 days gives the exact date, and the Countdown mode shows the remaining weeks and days live.
What week of the year is it right now?
The live widget at the top of this page shows the current ISO week number and a progress bar tracking how far through the current week and year you are. The Week Number mode also shows today's week number, the Monday–Sunday range, the day of the week, and how many weeks remain in the year.
Can I use this for pregnancy due date calculation?
Yes. The Pregnancy mode uses Naegele's rule (LMP + 280 days) to estimate the due date, shows all standard milestone dates plus a baby-size comparison for each week, and exports the due date as an .ics calendar event. It is an estimate only — ultrasound dating and your clinician's assessment take precedence.
How do I add the countdown to my calendar?
In the Countdown mode, click the 'Add to calendar' button. It downloads an .ics file you can open in Google Calendar, Apple Calendar, or Outlook. The event is created on the target date as an all-day event.
How many weeks is a federal tax year?
In the US and most countries the tax year runs the full calendar year — 52 weeks plus 1 or 2 days. The UK tax year runs from 6 April to 5 April, which is 52 weeks and 2 days in most years. Australia's tax year runs 1 July to 30 June.
How many weeks until my baby is due?
Use the Pregnancy mode: enter your LMP and the tool shows your current gestational age in weeks and days, plus the exact number of days until your due date (LMP + 280 days). Divide days remaining by 7 to get the number of weeks until birth.
What week of pregnancy do you find out the gender?
Blood tests (NIPT) can reveal fetal sex from about 10 weeks. Ultrasound can often identify sex from around 18–20 weeks at the anatomy scan. The tool's milestone timeline includes both markers.
How do I calculate how many weeks into the year we are?
Divide the current ISO week number by the total ISO weeks in the year (52 or 53) to get a percentage. The live widget at the top of this page shows both the week number and the percentage through the year. Week Number mode shows the exact ISO week number for any date.
What is a bi-weekly pay period in weeks?
A bi-weekly pay period is exactly 2 weeks (14 days). There are 26 bi-weekly pay periods in a year: 26 × 14 = 364 days, so one or two days are added to a particular period each year. Use the Sprint mode or Add/Subtract mode to project pay dates 2 weeks apart.
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Sources and methodology
- ISO 8601 — Date and time format: the international standard for week numbering, Monday-start weeks, and the 4-January rule.
- timeanddate.com — Week Number Calculator: cross-reference for ISO week lookup and year boundaries.
- NIST Time and Frequency Division: authoritative reference for calendar arithmetic and leap-year rules.
- Naegele's rule for pregnancy dating: LMP + 280 days. Standard obstetric convention; see ACOG Committee Opinion on gestational age dating.
- Baby-size comparisons drawn from common pregnancy tracker conventions (What to Expect, BabyCenter, NHS Start4Life). Lengths are crown-to-rump until week 20, then crown-to-heel.
- US Treasury bill maturities: 4, 8, 13, 17, 26, and 52 weeks — US Department of the Treasury, TreasuryDirect.
- Scrum sprint lengths: 1–4 weeks per the Scrum Guide (Schwaber & Sutherland).
- Competitor analysis: DQYDJ Week Calculator, Omnicalculator Week Calculator, timeanddate Week Number, WeeksCalculatorPro, datetimecalculator.io, Calculatorov, EpochPilot, CalculatorHub, Calcoid. Used for feature analysis only; no wording, code, or layout copied.