Fraction to MM Chart
Every inch fraction from 1/64" to 1" converts to an exact millimetre value. 1/2" = 12.7 mm exactly — this chart lists all 64 graduations plus how close each one sits to a round millimetre number.
Inch Fractions Rarely Land on Round MM
Because an inch is 25.4 mm — not a round multiple of 64 — almost none of the 64 standard inch fractions convert to a tidy whole millimetre value.
That matters in practice whenever a fractional-inch part needs a metric substitute: a 1/2" shaft is 12.7 mm, close to but not exactly a 13 mm metric stock size. This chart's "nearest whole mm" and "difference" columns show exactly how big that gap is for every fraction, so you can judge whether a metric substitute is close enough for the job.
All 64 Graduations in Millimetres
| Fraction | Exact MM | Nearest whole mm | Difference |
|---|---|---|---|
| 1/64" | 0.3969 mm | 0 mm | +0.3969 mm |
| 1/32" | 0.7937 mm | 1 mm | -0.2063 mm |
| 3/64" | 1.1906 mm | 1 mm | +0.1906 mm |
| 1/16" | 1.5875 mm | 2 mm | -0.4125 mm |
| 5/64" | 1.9844 mm | 2 mm | -0.0156 mm |
| 3/32" | 2.3812 mm | 2 mm | +0.3812 mm |
| 7/64" | 2.7781 mm | 3 mm | -0.2219 mm |
| 1/8" | 3.175 mm | 3 mm | +0.175 mm |
| 9/64" | 3.5719 mm | 4 mm | -0.4281 mm |
| 5/32" | 3.9688 mm | 4 mm | -0.0313 mm |
| 11/64" | 4.3656 mm | 4 mm | +0.3656 mm |
| 3/16" | 4.7625 mm | 5 mm | -0.2375 mm |
| 13/64" | 5.1594 mm | 5 mm | +0.1594 mm |
| 7/32" | 5.5562 mm | 6 mm | -0.4438 mm |
| 15/64" | 5.9531 mm | 6 mm | -0.0469 mm |
| 1/4" | 6.35 mm | 6 mm | +0.35 mm |
| 17/64" | 6.7469 mm | 7 mm | -0.2531 mm |
| 9/32" | 7.1437 mm | 7 mm | +0.1437 mm |
| 19/64" | 7.5406 mm | 8 mm | -0.4594 mm |
| 5/16" | 7.9375 mm | 8 mm | -0.0625 mm |
| 21/64" | 8.3344 mm | 8 mm | +0.3344 mm |
| 11/32" | 8.7312 mm | 9 mm | -0.2688 mm |
| 23/64" | 9.1281 mm | 9 mm | +0.1281 mm |
| 3/8" | 9.525 mm | 10 mm | -0.475 mm |
| 25/64" | 9.9219 mm | 10 mm | -0.0781 mm |
| 13/32" | 10.3187 mm | 10 mm | +0.3187 mm |
| 27/64" | 10.7156 mm | 11 mm | -0.2844 mm |
| 7/16" | 11.1125 mm | 11 mm | +0.1125 mm |
| 29/64" | 11.5094 mm | 12 mm | -0.4906 mm |
| 15/32" | 11.9063 mm | 12 mm | -0.0938 mm |
| 31/64" | 12.3031 mm | 12 mm | +0.3031 mm |
| 1/2" | 12.7 mm | 13 mm | -0.3 mm |
| 33/64" | 13.0969 mm | 13 mm | +0.0969 mm |
| 17/32" | 13.4937 mm | 13 mm | +0.4937 mm |
| 35/64" | 13.8906 mm | 14 mm | -0.1094 mm |
| 9/16" | 14.2875 mm | 14 mm | +0.2875 mm |
| 37/64" | 14.6844 mm | 15 mm | -0.3156 mm |
| 19/32" | 15.0812 mm | 15 mm | +0.0812 mm |
| 39/64" | 15.4781 mm | 15 mm | +0.4781 mm |
| 5/8" | 15.875 mm | 16 mm | -0.125 mm |
| 41/64" | 16.2719 mm | 16 mm | +0.2719 mm |
| 21/32" | 16.6687 mm | 17 mm | -0.3313 mm |
| 43/64" | 17.0656 mm | 17 mm | +0.0656 mm |
| 11/16" | 17.4625 mm | 17 mm | +0.4625 mm |
| 45/64" | 17.8594 mm | 18 mm | -0.1406 mm |
| 23/32" | 18.2562 mm | 18 mm | +0.2562 mm |
| 47/64" | 18.6531 mm | 19 mm | -0.3469 mm |
| 3/4" | 19.05 mm | 19 mm | +0.05 mm |
| 49/64" | 19.4469 mm | 19 mm | +0.4469 mm |
| 25/32" | 19.8438 mm | 20 mm | -0.1563 mm |
| 51/64" | 20.2406 mm | 20 mm | +0.2406 mm |
| 13/16" | 20.6375 mm | 21 mm | -0.3625 mm |
| 53/64" | 21.0344 mm | 21 mm | +0.0344 mm |
| 27/32" | 21.4312 mm | 21 mm | +0.4312 mm |
| 55/64" | 21.8281 mm | 22 mm | -0.1719 mm |
| 7/8" | 22.225 mm | 22 mm | +0.225 mm |
| 57/64" | 22.6219 mm | 23 mm | -0.3781 mm |
| 29/32" | 23.0187 mm | 23 mm | +0.0187 mm |
| 59/64" | 23.4156 mm | 23 mm | +0.4156 mm |
| 15/16" | 23.8125 mm | 24 mm | -0.1875 mm |
| 61/64" | 24.2094 mm | 24 mm | +0.2094 mm |
| 31/32" | 24.6062 mm | 25 mm | -0.3938 mm |
| 63/64" | 25.0031 mm | 25 mm | +0.0031 mm |
| 1" | 25.4 mm | 25 mm | +0.4 mm |
Frequently Asked Questions
What is 1/4 inch in mm on this chart?
1/4" equals 6.35 mm, found by multiplying 0.25 by 25.4.
Why doesn't the mm column show round numbers?
Because 25.4 isn't a round multiple of 64, so almost none of the 64 fractional-inch steps land exactly on a whole millimetre. Only 1 inch itself (25.4 mm) and a handful of other fractions come close to a round number.
What's the closest metric drill size to 1/2 inch?
1/2" equals exactly 12.7 mm, which rounds to 13 mm — the value shown in the "nearest whole mm" column, with a difference of +0.3 mm.
How much does each 64th step change in mm?
Each successive sixty-fourth adds exactly 25.4 ÷ 64 = 0.396875 mm to the previous value — a small, constant step across the entire chart.
Can I use this chart to pick a substitute metric part?
Roughly, yes — matching a fractional-inch part to its nearest whole-millimetre size is a common shop trick when an exact metric equivalent isn't available. The "difference" column shows exactly how far off that substitution would be, which can run up to about 0.4 mm at the largest gaps, so always confirm the actual tolerance the application needs.
Reference Charts
Every other chart in this cluster, one click away.