Inch Fraction to MM Converter
Inch fraction to convert
Enter an inch fraction, such as 17 over 64, for its exact millimetre equivalent.
Millimetres
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To convert an inch fraction to millimetres, divide the numerator by the denominator, then multiply by 25.4. 17/64" = 0.265625", and 0.265625 × 25.4 = 6.746875 mm.
Fine Fractions, Exact Millimetres
A machinist's rule or a set of number and letter drill bits often specifies sizes as fine inch fractions — 64ths and beyond — and those need to convert to millimetres with every digit intact, not rounded off.
Because 25.4 is an exact constant, an inch fraction like 17/64" doesn't just get an approximate millimetre value — it gets an exact one, 6.746875 mm, calculated to as many decimal places as the fraction supports. That precision matters when cross-referencing a fractional drill size against a metric one, covered on this site's drill size charts.
The Fraction to MM Formula
Two steps carry any inch fraction to an exact millimetre value.
V is the fraction's decimal value. MM is that decimal multiplied by the exact 25.4 mm-per-inch constant.
- NThe numerator of the inch fraction. In 17/64", N = 17.
- DThe denominator of the inch fraction. In 17/64", D = 64.
- VThe decimal inch value, N ÷ D.
- MMThe exact millimetre value, V × 25.4.
Worked calculation — 17/64"
- 17 ÷ 64 = 0.265625Divide the numerator by the denominator
- 0.265625 × 25.4 = 6.746875Multiply by the exact constant
17/64" = 6.746875 mm
Sixty-Fourths of an Inch in Millimetres
Fine inch fractions up to 1/2", the range most often used for drill and tap sizing.
| Fraction | Millimetres |
|---|---|
| 1/64" | 0.396875 mm |
| 1/32" | 0.79375 mm |
| 3/64" | 1.190625 mm |
| 1/16" | 1.5875 mm |
| 5/64" | 1.984375 mm |
| 3/32" | 2.38125 mm |
| 1/8" | 3.175 mm |
| 5/32" | 3.96875 mm |
| 3/16" | 4.7625 mm |
| 7/32" | 5.55625 mm |
| 1/4" | 6.35 mm |
| 5/16" | 7.9375 mm |
| 3/8" | 9.525 mm |
| 7/16" | 11.1125 mm |
| 1/2" | 12.7 mm |
Converting Without a Calculator
- Divide the numerator by the denominator. 5/32 → 5 ÷ 32 = 0.15625.
- Multiply the result by 25.4. 0.15625 × 25.4 = 3.96875 mm.
Frequently Asked Questions
What is 1/32 inch in mm?
1/32" equals 0.79375 mm, found by dividing 25.4 by 32.
How do you convert a machinist fraction like 17/64" to mm?
Divide the numerator by the denominator to get a decimal inch, then multiply by 25.4. For 17/64": 17 ÷ 64 = 0.265625", and 0.265625 × 25.4 = 6.746875 mm.
Is inch-fraction-to-mm conversion ever inexact?
No — since 25.4 is an exact defined constant, multiplying any exact fraction's decimal value by it always gives an exact millimetre result, even though the digits can run long, as with 17/64" = 6.746875 mm.
What is 9/16 inch in mm?
9/16" equals 14.2875 mm, found by dividing 9 by 16 to get 0.5625", then multiplying by 25.4.
Why do metric drawings need this conversion?
A drawing produced to a metric standard needs every dimension in millimetres, including any that originated from an imperial part or a legacy tape-measure spec. Converting the inch fraction directly to mm keeps the dimension exact rather than re-measuring the part from scratch.
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