Fraction of an inch to mm chart: 1/64 to 1 inch
Every 64th of an inch from 1/64 to 1, reduced to its simplest form, which is how drill bits, wrenches and tape measures are labeled. Find the fraction on the tool and read across.
| Fraction | Decimal inches | Millimeters |
|---|---|---|
| 1/64 in | 0.015625 | 0.396875 |
| 1/32 in | 0.031250 | 0.793750 |
| 3/64 in | 0.046875 | 1.190625 |
| 1/16 in | 0.062500 | 1.587500 |
| 5/64 in | 0.078125 | 1.984375 |
| 3/32 in | 0.093750 | 2.381250 |
| 7/64 in | 0.109375 | 2.778125 |
| 1/8 in | 0.125000 | 3.175000 |
| 9/64 in | 0.140625 | 3.571875 |
| 5/32 in | 0.156250 | 3.968750 |
| 11/64 in | 0.171875 | 4.365625 |
| 3/16 in | 0.187500 | 4.762500 |
| 13/64 in | 0.203125 | 5.159375 |
| 7/32 in | 0.218750 | 5.556250 |
| 15/64 in | 0.234375 | 5.953125 |
| 1/4 in | 0.250000 | 6.350000 |
| 17/64 in | 0.265625 | 6.746875 |
| 9/32 in | 0.281250 | 7.143750 |
| 19/64 in | 0.296875 | 7.540625 |
| 5/16 in | 0.312500 | 7.937500 |
| 21/64 in | 0.328125 | 8.334375 |
| 11/32 in | 0.343750 | 8.731250 |
| 23/64 in | 0.359375 | 9.128125 |
| 3/8 in | 0.375000 | 9.525000 |
| 25/64 in | 0.390625 | 9.921875 |
| 13/32 in | 0.406250 | 10.318750 |
| 27/64 in | 0.421875 | 10.715625 |
| 7/16 in | 0.437500 | 11.112500 |
| 29/64 in | 0.453125 | 11.509375 |
| 15/32 in | 0.468750 | 11.906250 |
| 31/64 in | 0.484375 | 12.303125 |
| 1/2 in | 0.500000 | 12.700000 |
| 33/64 in | 0.515625 | 13.096875 |
| 17/32 in | 0.531250 | 13.493750 |
| 35/64 in | 0.546875 | 13.890625 |
| 9/16 in | 0.562500 | 14.287500 |
| 37/64 in | 0.578125 | 14.684375 |
| 19/32 in | 0.593750 | 15.081250 |
| 39/64 in | 0.609375 | 15.478125 |
| 5/8 in | 0.625000 | 15.875000 |
| 41/64 in | 0.640625 | 16.271875 |
| 21/32 in | 0.656250 | 16.668750 |
| 43/64 in | 0.671875 | 17.065625 |
| 11/16 in | 0.687500 | 17.462500 |
| 45/64 in | 0.703125 | 17.859375 |
| 23/32 in | 0.718750 | 18.256250 |
| 47/64 in | 0.734375 | 18.653125 |
| 3/4 in | 0.750000 | 19.050000 |
| 49/64 in | 0.765625 | 19.446875 |
| 25/32 in | 0.781250 | 19.843750 |
| 51/64 in | 0.796875 | 20.240625 |
| 13/16 in | 0.812500 | 20.637500 |
| 53/64 in | 0.828125 | 21.034375 |
| 27/32 in | 0.843750 | 21.431250 |
| 55/64 in | 0.859375 | 21.828125 |
| 7/8 in | 0.875000 | 22.225000 |
| 57/64 in | 0.890625 | 22.621875 |
| 29/32 in | 0.906250 | 23.018750 |
| 59/64 in | 0.921875 | 23.415625 |
| 15/16 in | 0.937500 | 23.812500 |
| 61/64 in | 0.953125 | 24.209375 |
| 31/32 in | 0.968750 | 24.606250 |
| 63/64 in | 0.984375 | 25.003125 |
| 1 in | 1.000000 | 25.400000 |
Both columns are exact, not rounded: no 64th of an inch needs more than six decimal places, in inches or in millimeters.
The inches to mm formula
mm = inches × 25.4from a fraction: inches = top ÷ bottom, then × 25.4
Divide the fraction out, then multiply by 25.4. 3/8 is 3 ÷ 8 = 0.375, and 0.375 × 25.4 = 9.525 mm. With a whole number in front, add it before multiplying: 1 1/4 in is 1.25 in, so 31.75 mm.
What is exact and what is rounded
The factor is exact by definition, and every tape measure fraction is a whole number of 64ths, so each one is an exact number of millimeters with at most six decimals. Nothing recurs, and nothing in the chart above is rounded. The rounding comes afterward, when a metric size is picked to stand in for the result: 3/4 in is exactly 19.05 mm, and the 19 mm part sold in its place is 0.05 mm smaller. A decimal that is not a 64th, such as 0.3333 in, is only as good as the digits typed. Before 1959 the US inch was very slightly longer, 25.40005 mm, a difference of two parts in a million that only old survey and gauge work would show.
Worked examples
A 3/8 in drill bit
3 ÷ 8 = 0.375 in. 0.375 × 25.4 = 9.525 mm. The nearest metric bits are 9.5 mm, which is 0.025 mm smaller, and 10 mm, which is 0.475 mm larger.
A 1 1/4 in fitting
1 + 1 ÷ 4 = 1.25 in. 1.25 × 25.4 = 31.75 mm. A 32 mm part is 0.25 mm bigger, a quarter of a millimeter that decides whether it slides or jams.
A 5/16 in bolt and an 8 mm hole
5 ÷ 16 = 0.3125 in. 0.3125 × 25.4 = 7.9375 mm, so the bolt is 0.0625 mm thinner than the hole and passes through with almost no play.
Where inches to mm goes wrong
Cutting the fraction short
3/8 is 0.375, not 0.38 and not 0.4. Multiply 0.38 by 25.4 and you get 9.652 mm, 0.127 mm over. Rounded twice, 0.4 gives 10.16 mm, more than half a millimeter out on a part under 10 mm.
Treating a metric size as its inch neighbor
6 mm is not 1/4 in. A quarter inch is 6.35 mm, so a 1/4 in pin will not enter a 6 mm hole and a 6 mm pin rattles in a 1/4 in one. The same goes for 12 mm and 1/2 in, which are 0.7 mm apart.
Converting a trade name instead of a measurement
Lumber and pipe are sold by nominal sizes, names that are larger than or different from the real dimensions. Multiplying the name by 25.4 gives a number that matches nothing. Put a caliper or a tape on the part and convert what it reads.
Questions about inches to mm
How many mm is 1/8 inch?
1/8 in is 3.175 mm. The usual metric stand-in is 3 mm, which is 0.175 mm smaller. Halving it, 1/16 in is 1.5875 mm.
How many mm is 1 inch?
Exactly 25.4 mm. The figure was agreed in 1959 by the national standards laboratories of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, and it has not changed since.
Is 5/16 inch the same as 8 mm?
Not quite. 5/16 in is 7.9375 mm, which is 0.0625 mm less than 8 mm. That is less than the clearance wrenches are made with: the ASME table of wrench openings puts a 5/16 in wrench at 0.315 to 0.321 in, which is 8.00 to 8.15 mm, so a 5/16 in wrench will usually go on an 8 mm head, and an 8 mm wrench sits a little looser on a 5/16 in head. For a drilled hole or a pin the 0.06 mm still counts.
What fraction of an inch is 0.375?
Multiply by 64 to get 64ths: 0.375 × 64 = 24, and 24/64 reduces to 3/8. So 0.375 in is 3/8 in, 9.525 mm. When the product is not a whole number, the decimal is not an exact 64th and the nearest one is a rounding.
How do I convert thousandths of an inch to mm?
Multiply the thousandths by 0.0254. Five thousandths, written 0.005 in, is 0.127 mm, and ten thousandths is 0.254 mm.
Sources
The numbers and rules on this page come from the documents below. Each link opens the original, last checked on October 7, 2026.
- National Bureau of Standards, Refinement of Values for the Yard and the Pound, Federal Register, July 1, 1959 records that the yard was 3600/3937 meter before July 1, 1959 and 0.9144 meter after, which is the change from the old US inch to 25.4 mm.
- NIST Handbook 44 (2026 edition), Appendix C: General Tables of Units of Measurement lists 1 inch as 2.54 centimeters (exactly), so each 64th in the chart is an exact decimal.
- Engineers Edge, Wrench Openings Table for Hex and Square Nuts per ASME B18.2.2 reproduces the table of wrench openings from that standard, in which the 5/16 in wrench has a minimum opening of 0.315 in and a maximum of 0.321 in.
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