MM to Inches Fraction Calculator
Turn a metric measurement into the nearest fraction of an inch you can find on a tape measure or drill set, and see how far off the fraction is.
More mm and inch tools
How to use the MM to Inches Fraction Calculator
- Enter the measurement in millimetres.
- Choose the precision — 1/16 inch for most tape measures.
- Read the fraction, the exact decimal inches and the rounding error.
How it works
The millimetres are divided by 25.4 to get decimal inches, multiplied by the chosen denominator, rounded to a whole number and simplified. The rounding error is the difference between that fraction and the exact size.
How to convert mm to an inch fraction
- Convert to decimal inches: divide the millimetres by 25.4. 10 mm ÷ 25.4 = 0.3937″.
- Multiply by the fraction you can measure — 16 for sixteenths: 0.3937 × 16 = 6.3.
- Round to a whole number and write it over 16: 6/16.
- Simplify: 6/16 = 3/8″. The rounding error is 0.375 − 0.3937 = −0.019″, about −0.48 mm.
The calculator above does all four steps and lets you choose 1/8, 1/16, 1/32 or 1/64 inch. Choose finer precision when the error matters.
MM to inch fractions table
| Millimetres | Decimal inches | Nearest 1/16″ | Error at 1/16″ | Nearest 1/64″ |
|---|---|---|---|---|
| 1 mm | 0.0394″ | 1/16″ | +0.59 mm | 3/64″ |
| 2 mm | 0.0787″ | 1/16″ | −0.41 mm | 5/64″ |
| 3 mm | 0.1181″ | 1/8″ | +0.18 mm | 1/8″ |
| 4 mm | 0.1575″ | 3/16″ | +0.76 mm | 5/32″ |
| 5 mm | 0.1969″ | 3/16″ | −0.24 mm | 13/64″ |
| 6 mm | 0.2362″ | 1/4″ | +0.35 mm | 15/64″ |
| 7 mm | 0.2756″ | 1/4″ | −0.65 mm | 9/32″ |
| 8 mm | 0.315″ | 5/16″ | −0.06 mm | 5/16″ |
| 9 mm | 0.3543″ | 3/8″ | +0.53 mm | 23/64″ |
| 10 mm | 0.3937″ | 3/8″ | −0.48 mm | 25/64″ |
| 11 mm | 0.4331″ | 7/16″ | +0.11 mm | 7/16″ |
| 12 mm | 0.4724″ | 1/2″ | +0.7 mm | 15/32″ |
| 14 mm | 0.5512″ | 9/16″ | +0.29 mm | 35/64″ |
| 15 mm | 0.5906″ | 9/16″ | −0.71 mm | 19/32″ |
| 16 mm | 0.6299″ | 5/8″ | −0.13 mm | 5/8″ |
| 18 mm | 0.7087″ | 11/16″ | −0.54 mm | 45/64″ |
| 20 mm | 0.7874″ | 13/16″ | +0.64 mm | 25/32″ |
| 22 mm | 0.8661″ | 7/8″ | +0.23 mm | 55/64″ |
| 25 mm | 0.9843″ | 1″ | +0.4 mm | 63/64″ |
| 30 mm | 1.1811″ | 1 3/16″ | +0.16 mm | 1 3/16″ |
The error column shows how far the 1/16″ mark is from the true size. Anything under about ±0.4 mm is usually fine for carpentry; for metalwork, use 1/64″ or the decimal value.
When is a fraction close enough?
Tape measures are marked in 1/16″ (1.5875 mm), so a fraction can be off by up to half of that — about 0.8 mm — before rounding to the next mark. That is fine for framing, shelving and fabric. For anything with a tolerance, such as drilling a hole for a metric bolt or fitting a bearing, use the exact decimal from the mm to inches converter or work in millimetres throughout.
For a printable list of every millimetre from 1 to 100 in decimal inches, see the mm to inches chart. To turn any decimal inch value into a fraction, use the decimal to fraction inches calculator.
Frequently asked questions
What is 10 mm as an inch fraction?
About 3/8″ to the nearest 1/16 (exact: 0.394″), or 25/64″ to the nearest 1/64.
What is 6 mm in inches as a fraction?
About 1/4″ (exact 0.236″) — a 6 mm bit is slightly smaller than a 1/4″ bit.
What is 3 mm as a fraction of an inch?
About 1/8″ (exact 0.118″).
Can every mm value be written as an exact inch fraction?
No — most metric sizes fall between fraction marks, which is why the error is shown.