About this calculator
Drawings, tools and materials from different countries mix inch fractions and millimetres. Use the first mode to turn a fraction such as 1 3/8" into millimetres. Use the second mode to find the nearest inch fraction for a millimetre size — for example, to choose an imperial drill, spanner or bar size.
Formula
Fraction → mm
Decimal inches = whole + numerator ÷ denominator
mm = decimal inches × 25.4
1 inch is defined as exactly 25.4 mm.
mm → fraction
n = round(mm ÷ 25.4 × D)
where D is 8, 16, 32 or 64. The fraction n/D is then simplified (for example 24/64 becomes 3/8).
Worked example
- 1 3/8" = 1 + 0.375 = 1.375 in = 1.375 × 25.4 = 34.925 mm
- 35 mm = 1.37795 in. × 64 = 88.19 → 88/64 = 1 3/8" (34.925 mm, 0.075 mm smaller)
- 10 mm to the nearest 1/16": 0.3937 × 16 = 6.30 → 6/16 = 3/8"
Assumptions and limitations
- Rounding is to the nearest fraction, which can be larger or smaller than your size. Check which direction matters (for example, a bolt must pass through a hole).
References
- International yard and pound agreement (1959): 1 inch = 25.4 mm exactly.