Inches to mm: fraction conversion chart and converter
Published October 10, 2026 — the exact conversion, every fraction from 1/64 to 1 inch in millimetres, common sizes, millimetres to inches with the nearest fraction, and a converter that accepts fractions.
In short: 1 inch = 25.4 mm exactly. To go from inches to millimetres, multiply by 25.4; the other way, divide by 25.4. A fraction is converted after dividing: 3/8 inch = 0.375 × 25.4 = 9.525 mm. Values worth remembering: 1/4 = 6.35 mm, 1/2 = 12.7 mm, 3/4 = 19.05 mm, 1/64 = 0.397 mm.
1. Inches ↔ millimetres converter
Type a value in either field. Inches accept fractions: 3/8, 1 1/2 or 1-1/2.
2. Inch fractions in millimetres, 1/64 to 1 inch
Exact values, not rounded (1/64 inch = 0.396875 mm). Sixteenth fractions, the most common on drawings and tools, are in bold.
3. Whole inches and common sizes
The foot is 12 inches (304.8 mm) and the yard 36 inches (914.4 mm), also exact values.
4. Millimetres to inches, with the nearest fraction
Inches rounded to four decimals. The last column gives the deviation between the nearest 1/64 fraction and the millimetre dimension: at most ± 0.198 mm, half of a 1/64.
5. Conversion pitfalls
- Nominal sizes are not dimensions: a “1/2 inch” pipe or fitting does not measure 12.7 mm. A G 1/2 pipe thread has a 20.955 mm major diameter: see the G and NPT pipe thread chart.
- Rounding to a fraction changes the dimension: replacing 10 mm with 25/64 inch removes 0.078 mm. On a fit, that deviation often exceeds the tolerance: convert the dimension and its tolerance, not an approximate fraction.
- Mil, thou and micro-inch: 1 mil (or thou) = 0.001 inch = 25.4 µm; 1 µin = 0.0254 µm, the unit of US surface roughness, see the Ra roughness chart in µm and µin.
- Inch threads and gears: a UNC thread or a diametral-pitch gear does not swap with its “nearest” metric equivalent. For gears, m = 25.4 / DP, see the gear memo.
- In SOLIDWORKS, a document's units are set in Document Properties → Units: MMGS (millimetre, gram, second) or IPS (inch, pound, second). Switching systems converts how dimensions are displayed, not their value.
Printable version: the inch ↔ mm conversion charts as a PDF (A4, no sign-up), to pin up in the workshop or keep in a binder.
6. FAQ
How many mm is 1 inch?
Exactly 25.4 mm. Since the 1959 international agreement, the yard is 0.9144 m and the inch, 1/36 of a yard, is therefore 25.4 mm: it is not a rounded value.
How do you convert inches to mm?
Multiply by 25.4. For a fraction, divide first: 3/8 inch = 0.375 inch = 0.375 × 25.4 = 9.525 mm. For 1 1/2 inches: 1.5 × 25.4 = 38.1 mm.
How do you convert mm to inches?
Divide by 25.4: 10 mm = 0.3937 inch. The nearest 1/64 fraction is 25/64 (9.922 mm), which is 0.078 mm short.
How many mm are 1/4, 1/2 and 3/4 inch?
1/4 inch = 6.35 mm; 1/2 inch = 12.7 mm; 3/4 inch = 19.05 mm. These values are exact.
Why doesn't a 1/2 inch pipe or fitting measure 12.7 mm?
Because “1/2” is a designation there, not a dimension. A G 1/2 pipe thread has a 20.955 mm major diameter. Pipe and pipe-thread sizes are read from their charts, not with the inch-to-mm conversion.