This page converts; reading a mark on a ruler and adding measurements are other jobs, and they have their own guides: how to read fractions on a ruler and adding fractional inches. Enter the measurement in inches, as a whole number, fraction or mixed number, and choose “Inches to millimetres”. The result appears as a fraction of a millimeter and as a decimal. For the reverse, choose “Millimetres to inches” and, if you want a ruler mark, add the increment, for example 1/64.
Worked example: how many mm is 5/16 of an inch?
A socket wrench is marked 5/16 in and your catalog is in millimeters. What size is it?
- Write the factor as a fraction. 25.4 = 254/10 = 127/5.
- Multiply. 5/16 × 127/5 = 635/80. The 5 in the numerator and the 5 in the denominator cancel, leaving 127/16.
- Convert to a decimal. 127 ÷ 16 = 7.9375.
Result: 5/16 in = 127/16 mm = 7.9375 mm.
Check: that measurement is a little less than a third of an inch, and a third of 25.4 mm is about 8.5 mm. The result, 7.9 mm, falls just below that.
Fractional inches to millimeters chart
Every fraction with denominator 2, 4, 8, 16, 32 or 64 gives a terminating decimal in millimeters, so the values in the chart are not rounded. Sixteenths cover most rulers and drill bits; the rows from 1/64 to 7/32 help when you need finer steps.
| Inches | Millimeters | Inches | Millimeters |
|---|---|---|---|
| 1/64 in | 0.396875 mm | 3/8 in | 9.525 mm |
| 1/32 in | 0.79375 mm | 7/16 in | 11.1125 mm |
| 3/64 in | 1.190625 mm | 1/2 in | 12.7 mm |
| 1/16 in | 1.5875 mm | 9/16 in | 14.2875 mm |
| 3/32 in | 2.38125 mm | 5/8 in | 15.875 mm |
| 1/8 in | 3.175 mm | 11/16 in | 17.4625 mm |
| 5/32 in | 3.96875 mm | 3/4 in | 19.05 mm |
| 3/16 in | 4.7625 mm | 13/16 in | 20.6375 mm |
| 7/32 in | 5.55625 mm | 7/8 in | 22.225 mm |
| 1/4 in | 6.35 mm | 15/16 in | 23.8125 mm |
| 5/16 in | 7.9375 mm | 1 in | 25.4 mm |
If a printed chart shows 3.18 mm for 1/8 in, it has rounded 3.175 to two decimals. To add several measurements, convert each one with its full value and round only the total.
From millimeters to a fraction of an inch
Multiply the millimeters by 5/127, which is 1 ÷ 25.4 written as a fraction. For 10 mm you get 50/127 in, about 0.39370 in (rounded to five decimals), and that is almost never a ruler fraction. So you have to choose a mark and say which one. To the nearest 64th, 0.3937 × 64 = 25.1968, which rounds to 25, and the mark is 25/64 in.
That fraction is not exactly 10 mm. 25/64 in is 9.921875 mm, which is 0.078125 mm less, and the signed error is 25/64 − 50/127 = −25/8128 in. The negative sign means the mark sits below the measurement.
| Measurement | Nearest mark | Value of that mark | Difference |
|---|---|---|---|
| 6 mm | 15/64 in | 5.953125 mm | −0.046875 mm |
| 10 mm | 25/64 in | 9.921875 mm | −0.078125 mm |
| 12 mm | 15/32 in | 11.90625 mm | −0.09375 mm |
| 20 mm | 25/32 in | 19.84375 mm | −0.15625 mm |
In these four cases the mark sits below the measurement, and the difference never exceeds half of a 1/64 in mark, which is 0.1984375 mm. If you prefer to round to eighths or sixteenths, the difference grows; the round to a fraction tool explains how ties are resolved.
Mixed measurements, thirds and negatives
A measurement like 2 1/4 in is 9/4 in, and 9/4 × 127/5 = 1143/20 = 57.15 mm. Writing it as 2.25 or as 9/4 gives the same result. With a third, 1/3 × 127/5 = 127/15 mm, a repeating decimal, 8.46̅ mm. If you need the decimal expansion of another fraction, use fraction to decimal, and for the opposite step, decimal to fraction.
A minus sign is kept in the result: −1/2 in is −12.7 mm. It is useful for a shift in the opposite direction, such as a correction to a dimension, even though a physical length has no sign.
Calculation accuracy and measurement precision
The conversion is exact because 25.4 is a definition, not a measurement. That 3/8 in is 9.525 mm on paper does not mean the part measures that: a caliper that reads to 0.01 mm cannot tell 9.525 from 9.53, and a millimeter ruler only separates 9 from 10 mm. Record the value with the decimals your instrument supports.
Thread sizes, nominal pipe sizes and other trade designations do not match the physical size of the part, so check the manufacturer’s chart before converting those numbers.
If what you need is to read a mark on the ruler, the guide to reading fractions of an inch on a ruler shows how to count the divisions. To combine several measurements before converting, there is the guide to adding fractional inches. If the fixed ratio is not inches to millimeters, set it up with a rule of three; the other units are under conversions.
Sources
- NIST, “SI Units – Length”: the inch, derived from the value of the yard effective July 1, 1959, is exactly equivalent to 25.4 mm. nist.gov/pml/owm/si-units-length, accessed October 17, 2026.
Frequently asked questions
How do I convert a decimal measurement in inches, like 0.3 in, to millimeters?
Multiply by 25.4 to get 7.62 mm. That value is not a ruler mark, because 0.3 equals 3/10 and 10 is not a ruler denominator. The nearest mark in 64ths comes from 0.3 × 64 = 19.2, which rounds to 19/64 in.
How many millimeters are 3 feet?
It is 914.4 mm. One foot is 12 inches, which is 12 × 25.4 = 304.8 mm, and 3 × 304.8 = 914.4. Convert the feet to inches first by multiplying by 12, then enter that total in the converter. The result matches 0.9144 m, the length of a yard.
Is a 1/4-inch bolt the same as a 6 mm bolt?
No. A quarter inch is 6.35 mm and an M6 bolt has a nominal diameter of 6 mm, so they differ by 0.35 mm. That gap is enough to keep a part from one system from fitting properly in a hole made for the other. Before swapping one part for another, compare the two measurements in millimeters.