Inches to MM Converter — Accepts Fractions and Decimals

What this converter does

It turns a measurement in inches into millimetres. That one job sounds simple, but the inches you actually deal with in the real world come in two shapes: decimals like 0.625 and fractions like 5/8. Most online converters only accept the decimal, which means you have to do a division in your head before the tool will help you. This one accepts both, so a tape measure reading can go straight into the box.

It also runs the other direction. Click the swap button and the tool flips to millimetre-to-inch conversion, which is what you want when you are looking at a European spec sheet or a metric part number and trying to figure out what it corresponds to in American sizes.

The supporting content on the page covers the four places this conversion comes up most often: SAE versus metric sockets and wrenches (where close is not close enough), drill bit sizing, wire gauge, and screen diagonals. Each of those has its own reference table below, all in both systems.

How to use the converter

1. Type the inches. Either as a decimal (1.5, 0.625) or a fraction (1 1/2, 5/8, 3/32). The tool parses both formats, including the mixed-number style with a space (1 1/2) or a hyphen (1-1/2).
2. Read the mm result. The result panel shows the exact millimetre value plus the equivalent in centimetres, metres, and the decimal form of the same inches.
3. Click a fraction button. If your measurement is a common fraction, use the fraction grid instead of typing. Change the denominator dropdown to show quarters, eighths, sixteenths, thirty-seconds, or sixty-fourths.
4. Drag on the ruler. The dual-scale ruler shows the same physical length on both scales — inches on top, millimetres on the bottom. Click or drag anywhere on it to set the value.
5. Check the reference tables. Below the tool there are full tables for fractional inches, decimal inches, socket sizes, drill bits, wire gauges, screen sizes, and paper sizes.
6. Copy or share. Copy the result with one click, copy it in fraction form for a shopping list, or share a URL that reproduces the current conversion.

Why an inch is exactly 25.4 millimetres

Before 1959, the inch was not a fixed value. The British inch was defined against a physical bronze bar kept in London. The American inch was defined slightly differently against its own physical standard. On paper the difference was a few parts per million, but in precision manufacturing it caused real problems — a screw made to "one inch" in one country would not fit a hole made to "one inch" in another.

In 1959, delegations from six English-speaking countries met and agreed on one definition: one yard equals exactly 0.9144 metres. Because there are exactly three feet in a yard and twelve inches in a foot, this fixed the inch at exactly 0.0254 metres — which is exactly 25.4 millimetres.

This value is not an approximation. It is exact, by definition. That is why a proper inches-to-mm conversion never has a rounding error in the conversion itself — the only rounding happens when you choose how many decimal places to display.

Doing it by hand

You do not always need a browser. The conversion is one multiplication:

If your inches are a fraction, convert to decimal first. The formula for that is simple: numerator ÷ denominator, then add the whole number if the fraction is mixed.

💡 Fast estimation. If you are in a shop and just need a rough number, multiply the inches by 25 instead of 25.4. It is about 1.6% low, which is close enough for judging whether a bolt will fit through a hole. For anything precise, use 25.4.

Where the SAE and metric worlds collide

The single biggest source of frustration when converting inches to mm is the fact that the two fastener systems were invented independently, and their sizes do not line up. An SAE bolt of a given size does not correspond to a metric bolt of a similar size — they are close, but never exact.

The gap between the two is why a mechanic's toolbox has both sets of sockets. A 1/2 inch socket has an across-the-flats opening of 12.7 mm. The nearest metric socket is 13 mm. That 0.3 mm difference is small — about the thickness of three sheets of paper — but under the torque of a stuck bolt it is enough to round the corners of the head. Once a bolt head rounds, you cannot remove it with a wrench at all; you need a bolt extractor or a hacksaw.

The same logic applies in reverse. A 10 mm bolt head is 10 mm across. The closest SAE sizes are 3/8 inch (9.525 mm) and 7/16 inch (11.113 mm). Neither will fit a 10 mm head properly. The 3/8 is 0.475 mm too small and will not go on. The 7/16 is 1.113 mm too big and will slip.

⚠ Rule of thumb. If a fastener is tight, use the exact size. Substitutions on loose bolts sometimes work; on anything that requires force, they cause damage.

Working with fractional inches

If you grew up with a metric ruler, the fraction system can feel foreign. But it is the standard on every US tape measure and on most SAE tools, so anyone doing home improvement or working on American equipment encounters it constantly.

The fractions you will see on a standard tape measure are all powers of 2: 1/2, 1/4, 1/8, 1/16, and — on the more detailed tapes — 1/32 and 1/64. This means every inch is divided into 2, 4, 8, 16, 32, or 64 parts, and the tick marks are printed in descending size.

When you read a tape measure, the process is: whole inches first, then count the largest tick marks after them, then the next-largest, and so on. A reading like "7 and three of the small ticks, with two more of the smallest" would be written 7 3/16 + 1/32, or 7 7/32 in a simplified form.

If you have a fractional measurement and want to convert it to mm, the fastest path is to type the fraction into the input above. The tool parses it, converts it, and shows the result.

Decimal inches vs fractional inches

Both notations refer to the same physical length. The difference is cultural and context-specific.

• Woodworkers, carpenters, and DIYers use fractional inches. A tape measure reads 3/4 inch, and the fraction goes straight into a cutting list.
• Machinists, engineers, and CNC programmers use decimal inches. A tolerance of "0.001 in" is much easier to work with than "1/1000 in", and the arithmetic is cleaner.
• Plumbing uses fractions (1/2 inch pipe, 3/4 inch fitting) but with a twist: those are nominal sizes and do not match the actual pipe outside diameter.
• Photography and videography use decimals for sensor size (1.5 inch, 0.63 inch) but fractions for filter threads (52 mm is metric, but 2-inch eyepieces are common in telescopes).
• Screen sizes are always decimal inches, and always diagonal.

Neither system is more correct. The important thing is to know which one the specification you are reading uses, and to convert carefully at the boundary between the two.

Who actually needs this conversion

If you have ever looked at a part labelled "1/2 inch" and a specification in millimetres, you already know why this conversion exists. Here are the specific situations where it comes up most often.

• Buying tools and fasteners online. An American listing quotes inches; a European listing quotes millimetres. You need to know whether "1/2 inch" and "12 mm" are the same thing (they are not, but they are close).
• Working on imported equipment. A German car has metric fasteners. A Japanese motorcycle mixes metric and JIS-standard sizes. A US-made machine tool uses SAE. Knowing the exact conversion helps you pick the right socket the first time.
• Engineering drawings. A drawing from a US firm uses inches; a drawing from a European firm uses mm. Converting between them is a routine task.
• 3D printing. Most 3D printing software uses mm as its native unit. If you have a model in inches, converting before you slice saves time and prevents scale errors.
• DIY home improvement. A "1 by 4" board is nominally 1 inch × 4 inches but is actually 3/4 inch × 3-1/2 inches. If you are matching it to a metric part, the fractions matter.
• Plumbing. Copper pipe in the US is sized by nominal inch (1/2 in, 3/4 in), but the actual outside diameter is larger. Metric pipe is sized by actual outside diameter (15 mm, 22 mm). The two systems do not interchange.
• Fashion and jewellery. Ring sizes in the US are fractional inches of circumference; European sizes are millimetres of circumference. Converting between them is exactly this operation.
• Screen sizes. Almost every device is sold by inch diagonal, but the physical mm dimensions are what you measure when you are fitting it into a space.
• Paper sizes. A4 is defined in mm. US Letter is defined in inches. Printing across borders means converting both.
• Shooting and ammunition. Calibres mix units wildly: ".223 Remington" is inches, "5.56 × 45 mm" is metric, and they refer to the same cartridge.

Common mistakes to avoid

• Using 25 instead of 25.4. It seems close — 1.6% low — but over a long measurement the error adds up. 100 inches at 25 mm/inch is 2,500 mm; at the correct rate it is 2,540 mm. That 40 mm difference is more than an inch and a half.
• Treating a fractional reading as a decimal. "1 1/2 inches" is 1.5 inches, not 1.12 or 1.50-as-in-fifty-hundredths. The dot in "1.5" is a decimal point; the slash in "1/2" is a division.
• Forgetting that 1/16 and 0.0625 are the same thing. They are, but if you type "1/16" into a calculator without converting first, most calculators will give you a wrong answer or an error.
• Mixing up nominal and actual sizes. A "2×4" lumber piece is not 2 inches by 4 inches. A "1/2 inch" pipe is not 1/2 inch in outside diameter. Always check whether the size is nominal or actual.
• Assuming SAE and metric fasteners interchange. A 1/2 inch bolt head and a 13 mm bolt head are 0.3 mm different. On a tight fastener that is enough to strip it.
• Rounding before the last step. Convert 5/8 to 0.63 instead of 0.625 and you lose 0.127 mm. On a tolerance-critical part that matters. Keep full precision until the end.
• Confusing wire gauge with drill gauge. AWG (American Wire Gauge) is for wire. Numbered drill bits use a different numbering system. The two are not the same. A 10-gauge wire and a #10 drill bit have very different diameters.
• Using screen size as physical size. A 55-inch TV is 55 inches diagonally. Its width is about 48 inches and its height about 27 inches (depending on the aspect ratio). Converting 55 inches to 1,397 mm tells you the diagonal, not the width.

How precise should you be?

Match the precision of the output to the precision of the input. A ruler reading of "5/8 inch" is precise to about 1/32 of an inch. Converting it to 15.875 mm and showing six decimal places implies a precision the original measurement does not have. 15.88 mm is more honest.

A machinist working from a drawing that says "0.500 ± 0.001 inch" can safely convert to "12.700 ± 0.025 mm" and keep all three decimal places. The measurement itself carries that precision.

💡 Rule of thumb. Count the meaningful digits in your input. Show the same number of digits (or one more) in the output. If you typed 0.625, show 15.88 or 15.875. If you typed 0.6250, show 15.8750.

Privacy

All calculations run in your browser. Nothing you type is uploaded, logged, cached, or stored on a server. The recent conversions list is saved to your browser's local storage only, and can be cleared from the tool or by clearing browser data. There is no account, no login, and no network request after the page loads.

Limitations

• The tool parses fractions with denominators up to 64. Exotic fractions like 5/7 or 3/128 work mathematically but are not standard and are unlikely to show up on a tape measure.
• Mixed-number input is accepted in the form "1 1/2" or "1-1/2", but not "1 and 1/2" or "1 + 1/2".
• The ruler is drawn at pixel precision and is for visual reference only. For precise measurement, use a physical ruler or caliper.
• The wire gauge table shows solid wire diameters. Stranded wire has a larger overall diameter for the same gauge, because of the air gaps between strands.
• Historical units and non-international inches (like the old Spanish pulgada) are not supported. This tool uses the international inch defined by the 1959 agreement.
• The nearest-fraction suggestions in the reference tables use denominators up to 64. Very precise decimals like 0.618034 will not have a clean fraction match at that resolution.

Frequently asked questions

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