What this calculator does
Turn millimeters into inches, or inches into millimeters, using the exact factor. Millimeters are the working unit of engineering drawings, hardware, cameras and rainfall gauges in metric countries. A millimeter is one thousandth of a meter. In the United States the workshop unit is the inch, split into halving fractions such as 1/8 and 1/16. So drills, sockets, lumber and tubing are often sold in one system and specified in the other.
One inch is exactly 25.4 mm, by international agreement. To go from inches to millimeters, multiply by 25.4; to go from millimeters to inches, divide by 25.4. The reverse direction on this page also accepts decimal fractions, so 3/8 in is entered as 0.375.
Where it's used
Why this math matters outside the classroom:
- Drill bits. A 6 mm drill bit is about 0.236 in, just under 1/4 in.
- Rainfall. Gardeners reading a metric weather report see that 50 mm of rain is about 1.97 in.
- Jewelry beads. An 8 mm bead is about 0.31 in, close to 5/16 in (0.3125 in).
- Camera film. Film that is 35 mm wide is about 1.38 in wide.
- Wrench sizes. A 19 mm socket is about 0.748 in, very close to a 3/4 in socket.
Grade level and standards: Grades 5–6; Common Core State Standards for Mathematics 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the size. Convert a fraction to a decimal first: 5/16 = 0.3125.
- Choose millimeters → inches or inches → millimeters.
- Read the result and working. In the inches → mm direction, the table lists common fractional sizes from 1/16 in to 4 in.
What the inputs mean
- Value to convert
- A length in millimeters (mm → in) or decimal inches (in → mm).
- Direction
- Which unit you are converting from.
Formula and method
mm = in × 25.4 in = mm ÷ 25.4 ≈ mm × 0.0393700787402
In the formulas, mm is the length in millimeters and in is the length in inches. The international inch was set at exactly 25.4 mm in 1959. Before that, the US and UK inches differed by a few millionths, which mattered to precision machinists.
Dividing by 25.4 gives an endless decimal for most whole millimeters, so expect results like 0.3937… rather than tidy fractions.
Worked example
What is a 10 mm wrench in inches?
- Divide by the exact factor, because every inch holds 25.4 mm: 10 ÷ 25.4.
- 10 ÷ 25.4 = 0.3937007874…
Result: 10 mm is about 0.394 in, just over 3/8 in (0.375 in), so a 3/8-inch socket is too small for a 10 mm bolt head.
Understanding the result
Mental shortcut: 25 mm ≈ 1 in. Dividing by 25 instead of 25.4 overstates the inches by 1.6%, so 100 mm reads as 4 in when the exact value is 3.937 in. For in to mm, multiply by 25 and add 2%.
Handy sizes: 1/16 in = 1.5875 mm, 1/8 in = 3.175 mm, 1/4 in = 6.35 mm, 1/2 in = 12.7 mm. A4 paper is 210 × 297 mm, about 8.27 × 11.69 in; US Letter is 215.9 × 279.4 mm. Rainfall of 25.4 mm is exactly one inch of rain.
Pitfall: nominal sizes (the name a part is sold under) are not always real sizes. A "1/2-inch" water pipe or a 2×4 board is named by tradition, not by its measured dimension, so converting the name gives the wrong physical size.
Assumptions and limitations
- The inch is the international inch of exactly 25.4 mm.
- Fractions are entered as decimals.
- The result is decimal inches; it is not rounded to the nearest 1/16 or 1/32.
- Thread pitches (threads per inch versus mm pitch) are a reciprocal conversion and need separate handling.
Frequently asked questions
How many millimeters are in an inch?
Exactly 25.4.
Which fraction of an inch is closest to 6 mm?
6 mm is 0.2362 in. The nearest sixteenth is 1/4 in (6.35 mm). Working in 64ths, 15/64 in (about 5.95 mm) is closer, so a 15/64 in drill bit is a near substitute for a 6 mm one.
How do I turn a decimal inch into a fraction?
Multiply the decimal by 16 (or 32, or 64) and round to a whole number; that is the numerator. 0.394 × 16 = 6.3, so about 6/16 = 3/8 in.
How this calculator is checked
Method: International yard and pound agreement (1959): 1 in = 25.4 mm exactly. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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