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mm ⇄ inch fractions
Type in either box and the other follows. The fraction is snapped to the precision you pick, and the rounding error is shown in millimetres — because "about 3/4 in" is exactly how a part ends up a hair too big.
Convert
The card
| Fraction | mm | Fraction | mm |
|---|---|---|---|
| 1/16 | 1.6 | 9/16 | 14.3 |
| 1/8 | 3.2 | 5/8 | 15.9 |
| 3/16 | 4.8 | 11/16 | 17.5 |
| 1/4 | 6.35 | 3/4 | 19.05 |
| 5/16 | 7.9 | 13/16 | 20.6 |
| 3/8 | 9.5 | 7/8 | 22.2 |
| 7/16 | 11.1 | 15/16 | 23.8 |
| 1/2 | 12.7 | 1 | 25.4 |
Worth memorising: 1/4 in = 6.35 mm (not 6 — a 6 mm bit wobbles in a 1/4 in collet), and "3/4 in" plywood usually measures 18.2 mm — closer to 23/32.
Fraction card from https://sketchmethis.com/mm-to-inches — free, no signup.
Why the error line matters
19 mm is not 3/4 in — it is 3/4 in minus 0.05 mm. On one part that is nothing; across a carcase of parts, converted and cut and stacked, the nothings add up to a drawer that binds. The converter shows the error at every snap so you decide when it matters, instead of discovering it in the assembly.
The other classic: metric plans cut with imperial tooling and vice versa. A "12 mm" groove cut with a 1/2 in bit is 0.7 mm too wide — visibly sloppy on a shelf that should press-fit. Convert the tool, not just the drawing.
Reading inch fractions fast
All workshop fractions are 64ths in disguise: 1/2 = 32/64, 3/8 = 24/64, 5/16 = 20/64. To go up or down one 64th (0.4 mm), add or subtract one from the numerator on the 64 scale. Tape measures marked in 32nds get you to ±0.4 mm — about the real accuracy of a pencil line anyway.
Questions
Which snap should I use?
1/32 matches a decent tape measure and is the default. Use 1/64 for machine setups and 1/8 for rough carpentry. The error line tells you what the snap just cost, every time.
Can I type "1 3/4"?
Yes — "1 3/4", "1-3/4", "7/4" and "1.75" all parse. Decimal commas too.
Is a millimetre or an inch workflow better?
Whichever your tools are. The expensive mistakes come from mixing — a metric cut list built from an imperial plan carries sixteen tiny roundings by the time it reaches the saw, which is precisely why this page shows each one.
Sixteen conversions per project is fifteen too many.
Sketch Me This is a tool being built to keep a whole project in one unit system from sketch to cut list: describe or sketch the piece, get dimensioned drawings with the joinery already subtracted — metric or imperial, never both by accident.