Length Converter — Every Metric, Imperial, Nautical, Astronomy & Specialty Unit

What a length converter actually does

Weight gets away with one system — the kilogram, with the pound as a near-universal secondary. Length is different. Length is contested by three parallel systems that all refused to die: metric, imperial/US, and a scattered collection of specialist units that exist because they solve a specific problem better than metres do.

That is why a length converter is not the same as a weight converter. A weight converter only needs to know about kilograms and pounds. A length converter has to relate feet, yards, chains, furlongs, fathoms, nautical miles, light-years, parsecs, twips, and Didot points to each other — and to the metre, which is the common reference all modern units are measured against.

This converter keeps every one of those units visible at once. You enter a number, pick the unit it is measured in, and every other unit updates live. The whole point of showing them all is that you can compare six units in six seconds instead of six clicks. And because the To dropdown contains all 44 units, cross-category conversions — miles to kilometres, inches to centimetres, feet to metres — work without ever leaving the page.

How to use the converter

1. Type a number. Whole numbers, decimals, and comma-formatted numbers all work. The tool strips commas and spaces automatically, so 1,609.344 and 1609.344 behave the same way.
2. Pick the unit you are entering in. The From dropdown lists every unit in the currently active category. If you want to enter in a different category, click its tab first — Metric, Imperial & US, Nautical, Astronomy, or Specialty.
3. Read the grid. Every unit is on screen by default. The input unit is highlighted in purple. The current To unit is highlighted in green. Click any cell to copy that value to your clipboard.
4. Pick a specific To unit if you want one. The To dropdown is grouped by system and contains all 44 units. Miles to kilometres, feet to centimetres, and inches to millimetres are one click away.
5. Check the formula strip. It shows the exact calculation with the two unit sizes in metres and the ratio between them.
6. Look at the log ruler. Your length is plotted from 0.1 nanometre to a kiloparsec, so you can see the magnitude at a glance.
7. Watch for the centimetre ruler. When the value is under 5 metres, a ruler appears with millimetre and centimetre marks so you can see exactly where the length falls on a tape measure.
8. Compare with real objects. The comparison chart always includes at least one smaller and one larger object relative to your value. Click any row to load that length as your new value.
9. Set the precision. Zero through ten decimal places. Use fewer for everyday lengths, more for engineering.
10. Use the presets. Four groups cover human scale, everyday objects, travel and sport, and science and space.

Doing the math by hand — the magic numbers

You do not always have a browser open. Sometimes you need to work out a length conversion in your head, on paper, or in a spreadsheet. Every length conversion reduces to one of a small set of multiplication factors. Learn these and you can convert almost anything.

💡 Mental-math shortcut for miles and kilometres. Multiply the km value by 5 and divide by 8. A 40 km distance is 40 × 5 ÷ 8 = 25 miles. It is not exact (you get 24.855 miles with the true factor), but it is within 0.6% and easy to do in your head.

💡 Mental-math shortcut for metres and feet. Multiply the metres by 3 and add a tenth. So 20 m ≈ 66 ft (20 × 3 = 60, plus 6). The true answer is 65.617 ft, so this is off by less than 0.6%.

The metre — where it comes from

Before the French Revolution, France alone had over 250,000 different units of measurement. Every region, every guild, every trade had its own. A livre in Paris weighed more than a livre in Lyon. A pied (foot) in Bordeaux was not a pied in Marseille. Trade, taxes, and engineering were a constant mess of local rules.

In 1791 the French Academy of Sciences proposed a radical idea: a single, decimal, universal unit based on the size of the Earth itself. The metre was to be defined as one ten-millionth of the distance from the North Pole to the equator, along the meridian passing through Paris. Two astronomers, Jean-Baptiste Delambre and Pierre Méchain, spent seven years surveying the meridian arc between Dunkirk and Barcelona. Their measurement was heroic but slightly off — the Earth is not a perfect sphere — yet it fixed the metre for the next century.

The physical standard moved three times. In 1889 a platinum-iridium bar became the international prototype metre. In 1960 the definition shifted to a specific wavelength of krypton-86 light, which could be reproduced in any lab. Then in 1983 the metre was finally anchored to a fundamental constant: the distance light travels in a vacuum in 1/299,792,458 of a second. This definition is still current. Because the speed of light never changes, this metre can be reproduced anywhere in the universe to the same precision.

The imperial foot — a history of barleycorns and kings

The foot is older than the metre by thousands of years. Almost every ancient culture had one. The Roman foot was 29.6 cm. The Greek foot was 30.8 cm. The Egyptian royal foot was about 35 cm. The Saxons brought their own foot to England, and that Saxon foot is the ancestor of the modern one.

The inch was defined in the 12th century as the length of three barleycorns laid end to end. That sounds charming but caused enormous practical problems, because barleycorns vary. Over the following centuries the length was fixed to a physical standard — a brass bar kept at the Tower of London, then later a bar made of bronze.

For most of the 20th century the US and the UK had slightly different feet because they were defined against different physical standards. This was a headache for international trade — a screw thread cut to "one inch" in Britain did not match one made in America by a fraction of a millimetre. On 1 July 1959, after years of negotiation, six countries agreed on one definition: 1 yard = 0.9144 metres exactly, and therefore 1 foot = 0.3048 metres exactly. Every English-speaking country except the United States adopted it that day. The US adopted it shortly after, though its land surveyors kept using the older survey foot for another 63 years.

The US survey foot — finally retired in 2022

The United States had its own definition of the foot that predated the 1959 agreement. Called the US survey foot, it was defined as 1200/3937 metres, which works out to about 0.3048006096 metres. That is about two parts per million longer than the international foot — a difference of less than a millimetre over a kilometre.

Two parts per million sounds trivial, but over the length of a hundred-mile survey it becomes a foot of error. Re-recording every property deed in the country to match a new standard was not practical, so the US simply kept using the older survey foot for land surveying while using the international foot for everything else.

This split lasted until 31 December 2022, when the National Institute of Standards and Technology officially retired the US survey foot from federal use. All new surveys now use the international foot, though the older value still appears in historic maps and land records. If you see a measurement labelled "ftUS" or "US survey foot" in an old document, that is what it means.

Why navigation uses nautical miles

The nautical mile exists because the Earth is a sphere (very nearly), and you can navigate a sphere with angles. One nautical mile is defined as one minute of latitude along a meridian — that is, 1/60 of one degree of latitude. That works out to exactly 1,852 metres.

This makes nautical navigation astonishingly simple. If you sail due north at a speed of one nautical mile per hour (one knot) for one hour, your latitude changes by exactly one minute of arc. Charts are drawn with latitude and longitude marked off in degrees and minutes, and distances are marked off in nautical miles. Any sailor reading a chart can move between distance and angle without doing any arithmetic.

Aviation inherited the nautical mile from maritime navigation and has kept it. Air traffic control uses feet for altitude and nautical miles for horizontal distance, all over the world. Even aircraft that never fly over water use these units, because every other aviation system already does.

The related nautical units follow the same logic. A fathom is 6 feet (1.8288 m), originally the span of a man's outstretched arms, and still used for water depth on charts. A cable is 100 fathoms in the Imperial system or 120 fathoms in the US Navy system — a holdover from the days when a ship's anchor cable was manufactured in standard lengths.

Astronomical distances — when kilometres stop working

Past Earth's orbit, kilometres become useless. The Moon is 384,400 km away — we can count that. The Sun is 149.6 million km — getting unwieldy. The nearest star is 40 trillion km — you need a different kind of number.

Astronomers use three units, each appropriate to a different scale.

The astronomical unit (AU) is the mean Earth-Sun distance, exactly 149,597,870,700 metres. It is used throughout the solar system. Mercury orbits at 0.39 AU. Jupiter at 5.20 AU. Neptune at 30.07 AU. The Voyager 1 spacecraft, launched in 1977, is now more than 160 AU from Earth.

The light-year (ly) is the distance light travels in a vacuum in one Julian year (365.25 days). That equals 9,460,730,472,580,800 metres, or about 9.46 trillion kilometres. Despite the word "year", it measures distance, not time. The nearest star to the Sun — Proxima Centauri — is about 4.24 light-years away.

The parsec (pc) is the distance at which one astronomical unit subtends one arcsecond of angle. It is about 3.0857 × 10¹⁶ metres, or 3.26 light-years. The name is a contraction of "parallax arcsecond". When Earth moves from one side of its orbit to the other, nearby stars appear to shift against the fixed background. The angle of that shift is measured in arcseconds, and a star with a parallax of one arcsecond is one parsec away. This direct relationship between observation and calculation is why the parsec became the astronomer's favourite unit.

Typography units — small, precise, still in use

Typography has its own length units, and they survive because modern design software inherited them from physical typesetting. The PostScript point was standardised by Adobe in 1984 as exactly 1/72 inch. Twelve points make a pica, so a pica is exactly 1/6 inch. A twip is 1/20 of a point, and Microsoft Word still uses twips internally for measuring distances on the page.

Continental European typography used the Didot point, based on the old French inch, which is about 0.3759 mm. This is just slightly different from the PostScript point (0.3528 mm). The difference is small enough that few designers ever notice it, but it matters if you are converting a scanned European print layout into digital form.

Metric or imperial? A regional and industry guide

The choice between metric and imperial units is not always about which system you personally prefer — it depends on where you are and what industry you work in. Here is the practical reality.

Some industries override national habits. Aviation uses feet and nautical miles everywhere on Earth. Shipping uses nautical miles and fathoms. Precision engineering uses millimetres or thousandths of an inch regardless of country. Cycling uses kilometres. Athletics uses metres and miles for different events. Video streaming platforms quote screen sizes in inches worldwide.

Field guides — how each industry actually uses length

Cooking and baking

Recipes mix units more than any other consumer activity. A European cake recipe gives ingredient sizes in centimetres (a 20 cm tin) and weights in grams, while an American recipe uses inches (an 8-inch tin) and cups. If you are converting between the two, an 8-inch round tin is 20.3 cm, and a 9-inch tin is 22.9 cm. These are not interchangeable — a 9-inch tin holds about 25% more batter.

Kitchen measurements that appear as length include: the diameter of a cake tin, the thickness of a steak (2 cm = ¾ inch), the length of a loaf tin (23 cm = 9 inches), and the height of a layer cake. This tool handles all of them.

DIY and home improvement

Construction tolerances are usually quoted in millimetres or fractions of an inch. A "2×4" in the US is nominally 2 inches by 4 inches but is actually 1.5 inches by 3.5 inches after planing — 38 mm × 89 mm. Tile sizes come in both systems. A standard European tile is 30 cm × 30 cm; the US equivalent is 12 inches × 12 inches, which is 30.48 cm. Over a floor, that 0.48 cm difference adds up to a visible discrepancy at the edges of the room.

Pipes and fasteners are the other area where unit confusion causes real problems. A "½-inch" pipe is a nominal size — its actual outside diameter is about 21 mm. A "12 mm" pipe has an outside diameter of exactly 12 mm. These are not interchangeable, even though both are called "half-inch" or "12 mm" in casual conversation.

Sports and athletics

Running races are the cleanest example of unit conflict. A 5K is a metric race at 5,000 metres, but American races often quote it as 3.107 miles. A marathon is exactly 42.195 km by international agreement, which is 26.2187 miles. The mile run (1,609.34 m) is a specific imperial distance still contested at some US high-school meets.

Field dimensions mix units by sport. Soccer pitches are metric internationally (100–110 m long) but imperial in US high school and college sports (110–120 yards long). American football fields are imperial by tradition (100 yards, plus end zones). Cricket pitches are 22 yards (20.12 m), a number that survives specifically because a mile is exactly 80 chains.

Travel and navigation

Aviation is a permanent hybrid of units. Altitudes are always in feet, regardless of the country you are flying over. Horizontal distances are in nautical miles. Speeds are in knots (nautical miles per hour). Runway lengths are quoted in metres in Europe and feet in the US. This is why a pilot's altimeter has two readouts — feet and metres — depending on which country they are flying into.

For ground travel, most of the world uses kilometres for road distances. The US, UK, and a handful of smaller countries use miles. Baggage allowances are almost always quoted in centimetres plus kilograms worldwide. A "55 × 40 × 20 cm" carry-on bag is the international standard; the "22 × 14 × 9 inch" limit is the American equivalent, and the two are close but not identical.

Science and engineering

Every field of science has settled on metric — the SI system is the international standard for research, publishing, and lab equipment. Chemistry uses angstroms and nanometres for atomic and molecular scales. Biology uses micrometres for cells and nanometres for DNA. Astronomy uses AU, parsecs, and light-years. Engineering in most countries uses metres and millimetres, though aerospace engineering in the US still mixes feet and inches for legacy reasons.

One quirk: in the semiconductor industry, chip features are measured in nanometres even though the underlying wafers are 200 mm or 300 mm in diameter. A "5 nm process" refers to a chip feature size that is roughly 5 nanometres across, though modern marketing has drifted from the strict physical meaning.

Fashion, screens, and paper

Screen sizes are quoted in inches diagonally, worldwide. This is one of the few places where imperial dominates even in metric countries. A "15-inch laptop" is universally understood — 38.1 cm just does not catch on.

Clothing sizes are the messiest. European sizes are usually centimetre-based, US sizes are often in inches, and UK sizes are their own numerical system. A size 38 dress in Europe corresponds to a size 8 in the UK and a size 6 in the US. These numbers are not directly convertible through any simple formula — they are separate systems that must be looked up in a size chart.

Paper sizes are unique in that they are almost entirely metric. The ISO 216 series — A0, A1, A2, A3, A4, A5 — is defined in millimetres and used worldwide, except in the US, Canada, Mexico, and the Philippines, which still use US Letter and Legal. The single most common paper size worldwide is A4 at 210 × 297 mm, which works out to 8.27 × 11.69 inches.

Common mistakes when converting length

• Confusing feet with decimal feet. "5 feet 10 inches" is 5.8333 feet, not 5.10 feet. The dot means a fraction, not inches. This is the most common height-conversion error.
• Using 1,000 for the nautical mile. A nautical mile is 1,852 metres, not a round metric number. It is defined by latitude, not by a prefix.
• Rounding too early. Convert 5 km to miles and round to 3.1, then convert back, and you land on 4.99 km. You have lost 11 metres. Keep full precision in every intermediate step and round only at the end.
• Mixing up m (metre) and mi (mile). Both start with "m". In any travel or distance context, always check the symbol.
• Assuming all feet are the same. The international foot is 0.3048 m; the older US survey foot is 0.3048006096 m. The difference matters for large land surveys but not for everyday work.
• Thinking of "millimetre" as two m's. In "mm" the first m means milli (one-thousandth) and the second m means metre. The same letter does two different jobs.
• Reading a comma as a decimal separator. In most of Europe, "1,5 m" means one and a half metres. In the US, "1,5 m" is likely a typo. The tool strips both, so any format works.
• Mixing systems mid-calculation. Convert into your target unit once at the end, not after every step. Rounding errors compound in ways you will not notice until the total is wrong.

Precision, rounding, and how many digits to show

The tool displays results to a precision you set between zero and ten decimal places. Choosing the right precision is a judgement call, but here is the principle: show no more digits than your original measurement actually supports.

A carpenter's tape measure reads to the nearest sixteenth of an inch, about 1.6 mm. A laser distance meter reads to the nearest millimetre. A surveyor's total station reads to a fraction of a millimetre over hundreds of metres. A GPS receiver gives coordinates to within about a metre. Whatever precision the source measurement has, that is the precision you should carry through to the answer.

Showing "5.1234 m" when the original was 5.5 m (measured to the nearest half metre) is false precision. It implies accuracy the measurement does not have.

💡 Rule of thumb. Count the number of meaningful digits in your input. Show the same number of digits in the output. If you typed 5.5 (two significant figures), show two: 18 ft, not 18.0446 ft.

Glossary of length units

Every unit that appears in this converter, in alphabetical order.

• Å (angstrom) — 1 × 10⁻¹⁰ m. Not SI, but still common in chemistry for atomic radii and bond lengths.
• Admiralty mile — 1,853.184 m. Historical British naval mile, superseded by the international nautical mile.
• Astronomical unit (AU) — 149,597,870,700 m exactly. The mean Earth-Sun distance.
• Barleycorn — 1/3 inch, or 8.4667 mm. The original definition of the inch.
• Cable — 100 fathoms (Imperial) or 120 fathoms (US Navy). Nautical unit for distance at sea.
• Chain — 66 feet, or 20.1168 m. Invented by Edmund Gunter in 1620. A cricket pitch is one chain long.
• Centimeter (cm) — 1/100 m. Standard metric unit for everyday small lengths.
• Cubit — Originally elbow to fingertip; the Egyptian royal cubit was 0.524 m.
• Decameter (dam) — 10 m. Rarely used in everyday life.
• Decimeter (dm) — 0.1 m. Still used in some European contexts.
• Didot point — 0.37597 mm. Continental European typographic unit.
• Earth radius (R⊕) — 6,371 km. Used for satellite orbits and planetary scales.
• Ell — 45 inches, or 1.143 m. Historical cloth measure.
• Fathom (ftm) — 6 feet, or 1.8288 m. Water depth.
• Foot (ft) — 0.3048 m exactly. Imperial and US customary base unit.
• Furlong (fur) — 220 yards, or 201.168 m. One-eighth of a mile. Horse racing.
• Gigameter (Gm) — 1 × 10⁹ m. Rare; interplanetary scale in metric.
• Hand (hh) — 4 inches, or 0.1016 m. Horse height.
• Hectometer (hm) — 100 m. Occasional use in surveying.
• Inch (in) — 2.54 cm exactly. Imperial base for smaller measures.
• Kilometer (km) — 1,000 m. Road distances and travel.
• Kiloparsec (kpc) — 1,000 parsecs, or about 3,260 light-years. Galactic scale.
• League (lea) — 3 miles, or 4,828.032 m. Historical. In classical literature.
• Light-second (ls) — 299,792,458 m. The distance light travels in one second.
• Light-year (ly) — 9,460,730,472,580,800 m. Distance light travels in one Julian year.
• Megameter (Mm) — 1 × 10⁶ m. Rare.
• Meter (m) — SI base unit for length.
• Micrometer (µm) — 1 × 10⁻⁶ m. Cell biology, fibre diameters.
• Mile (mi) — 1,609.344 m exactly. Imperial long-distance unit.
• Millimeter (mm) — 1/1000 m. Engineering precision.
• Nanometer (nm) — 1 × 10⁻⁹ m. Chip fabrication, light wavelengths.
• Nautical mile (nmi) — 1,852 m exactly. One minute of latitude.
• Parsec (pc) — About 3.0857 × 10¹⁶ m, or 3.26 light-years.
• PostScript pica (P) — 12 points, or exactly 1/6 inch.
• PostScript point (pt) — 1/72 inch exactly. Universal digital typography.
• Roman pace (passus) — 1.48 m. Two steps.
• Rod (rd) — 16.5 feet, or 5.0292 m. Historical surveying.
• Solar radius (R☉) — 695,700 km. Used for star sizes.
• Span — 9 inches, or 0.2286 m. Width of an outstretched hand.
• Thou (mil) — 0.001 inch, or 0.0254 mm. Engineering tolerances.
• Twip — 1/1440 inch. Microsoft Word's internal unit.
• US Survey Foot (ftUS) — 1200/3937 m. Legacy surveying unit, retired December 2022.
• Yard (yd) — 0.9144 m exactly. Fabric, sports fields.

Privacy

Every calculation happens in your browser using vanilla JavaScript. Nothing you type is uploaded, logged, cached, or sent to any server. The recent conversions list lives only in your browser's local storage so that you can return to previous work — it can be cleared from the tool or by clearing your browser data. There is no account, no login, and no network request after the page loads.

Limitations

• The tool shows 44 units. Extremely specialised historical units — the Russian verst, the Japanese shaku, the Indian kos, the Spanish legua — are not included.
• Compound input such as "5 ft 10 in" is not accepted in one field. Enter such heights as decimal feet (5.8333) or convert inches separately.
• Values are rounded for display to your chosen precision. The underlying values retain full precision — copy a value or raise the precision to see more digits.
• The log-scale ruler stops at about 0.1 nm on the small end and one kiloparsec on the large end. Values outside that range still convert correctly, but the marker clamps to the edge.
• The centimetre ruler appears only for values between 1 mm and 5 m. Above and below that, use the log ruler and comparison chart.
• Historical units like the cubit, span, and ell are shown at their standard modern approximations. In antiquity they varied regionally and across centuries.

Frequently asked questions

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