Fractional Inches to mm Converter
Convert fractional inches like 3/8 or 1 1/2 to millimetres and back, with a filterable 1/32-inch chart.
Type in either box — the other one updates as you type. Inches accept whole numbers,
fractions (3/8), mixed numbers (1 1/2) and decimals (0.375).
- Millimetres
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- Centimetres
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- Decimal inches
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- Fraction
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- Fraction vs exact
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Fractional inch to mm chart
Every 1/32 inch up to 1 inch, using the exact conversion 1 in = 25.4 mm. Filter the chart to jump to a size.
No sizes match that filter.
| Fraction | Decimal inches | Millimetres |
|---|---|---|
| 1/32" | 0.03125 | 0.79375 |
| 1/16" | 0.0625 | 1.5875 |
| 3/32" | 0.09375 | 2.38125 |
| 1/8" | 0.125 | 3.175 |
| 5/32" | 0.15625 | 3.96875 |
| 3/16" | 0.1875 | 4.7625 |
| 7/32" | 0.21875 | 5.55625 |
| 1/4" | 0.25 | 6.35 |
| 9/32" | 0.28125 | 7.14375 |
| 5/16" | 0.3125 | 7.9375 |
| 11/32" | 0.34375 | 8.73125 |
| 3/8" | 0.375 | 9.525 |
| 13/32" | 0.40625 | 10.31875 |
| 7/16" | 0.4375 | 11.1125 |
| 15/32" | 0.46875 | 11.90625 |
| 1/2" | 0.5 | 12.7 |
| 17/32" | 0.53125 | 13.49375 |
| 9/16" | 0.5625 | 14.2875 |
| 19/32" | 0.59375 | 15.08125 |
| 5/8" | 0.625 | 15.875 |
| 21/32" | 0.65625 | 16.66875 |
| 11/16" | 0.6875 | 17.4625 |
| 23/32" | 0.71875 | 18.25625 |
| 3/4" | 0.75 | 19.05 |
| 25/32" | 0.78125 | 19.84375 |
| 13/16" | 0.8125 | 20.6375 |
| 27/32" | 0.84375 | 21.43125 |
| 7/8" | 0.875 | 22.225 |
| 29/32" | 0.90625 | 23.01875 |
| 15/16" | 0.9375 | 23.8125 |
| 31/32" | 0.96875 | 24.60625 |
| 1" | 1 | 25.4 |
How it works
The conversion is exact and fixed by the 1959 international yard and pound agreement:
1 inch = 25.4 mm. So millimetres = inches × 25.4, and inches = mm ÷ 25.4.
A fraction like 3/8 is first turned into a decimal (3 ÷ 8 = 0.375) and then
multiplied, giving 0.375 × 25.4 = 9.525 mm. Mixed numbers such as
1 1/2 add the whole part first: 1.5 × 25.4 = 38.1 mm.
Going the other way, millimetres rarely land on a tidy fraction, so the converter snaps the result to the nearest 1/8, 1/16, 1/32 or 1/64 inch — your choice — and tells you exactly how far the rounded fraction sits from the true value. If the value does land on a 64th (10 mm does not, but 12.7 mm does — that is precisely 1/2 inch), the exact fraction is shown instead and the difference reads “Exact”. Fractions are always reduced to lowest terms, so 8/16 is displayed as 1/2.
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Frequently asked questions
What is 3/8 of an inch in mm?
3/8 inch is 9.525 mm. One inch is exactly 25.4 mm, so you divide 3 by 8 to get 0.375 inches and multiply by 25.4: 0.375 x 25.4 = 9.525 mm. The same method gives 1/2 inch = 12.7 mm, 5/8 inch = 15.875 mm and 1 1/2 inches = 38.1 mm.
How do I convert mm back to a fractional inch?
Divide the millimetres by 25.4 to get decimal inches, then round to the nearest 1/8, 1/16, 1/32 or 1/64 and reduce the fraction. For example 10 mm is 0.3937 inches, which is 3/8 inch to the nearest sixteenth (0.475 mm smaller than the true value) or 25/64 inch to the nearest sixty-fourth (only 0.078 mm out). This tool does both and always shows the rounding error.
Is 1 inch exactly 25.4 mm?
Yes. Since the 1959 international yard and pound agreement the inch is defined as exactly 25.4 millimetres, so inch-to-mm conversions are exact rather than approximate. Millimetre values only become approximate in the other direction, when a metric measurement has to be rounded to the nearest usable fraction of an inch.