Metric drill and wrench sizes in fractional inches
Fractional drill bits step in 64ths of an inch and most wrench and socket sets in 16ths, so each metric size is matched against both. The difference columns show how much larger (+) or smaller (-) the inch size is.
| Millimeters | Inches | Nearest 1/64 in | 1/64 difference, mm | Nearest 1/16 in | 1/16 difference, mm |
|---|---|---|---|---|---|
| 1 mm | 0.0394 | 3/64 | +0.191 | 1/16 | +0.588 |
| 1.5 mm | 0.0591 | 1/16 | +0.088 | 1/16 | +0.088 |
| 2 mm | 0.0787 | 5/64 | -0.016 | 1/16 | -0.413 |
| 2.5 mm | 0.0984 | 3/32 | -0.119 | 1/8 | +0.675 |
| 3 mm | 0.1181 | 1/8 | +0.175 | 1/8 | +0.175 |
| 3.5 mm | 0.1378 | 9/64 | +0.072 | 1/8 | -0.325 |
| 4 mm | 0.1575 | 5/32 | -0.031 | 3/16 | +0.763 |
| 4.5 mm | 0.1772 | 11/64 | -0.134 | 3/16 | +0.263 |
| 5 mm | 0.1969 | 13/64 | +0.159 | 3/16 | -0.238 |
| 5.5 mm | 0.2165 | 7/32 | +0.056 | 3/16 | -0.738 |
| 6 mm | 0.2362 | 15/64 | -0.047 | 1/4 | +0.350 |
| 6.5 mm | 0.2559 | 1/4 | -0.150 | 1/4 | -0.150 |
| 7 mm | 0.2756 | 9/32 | +0.144 | 1/4 | -0.650 |
| 8 mm | 0.3150 | 5/16 | -0.063 | 5/16 | -0.063 |
| 9 mm | 0.3543 | 23/64 | +0.128 | 3/8 | +0.525 |
| 10 mm | 0.3937 | 25/64 | -0.078 | 3/8 | -0.475 |
| 11 mm | 0.4331 | 7/16 | +0.113 | 7/16 | +0.113 |
| 12 mm | 0.4724 | 15/32 | -0.094 | 1/2 | +0.700 |
| 13 mm | 0.5118 | 33/64 | +0.097 | 1/2 | -0.300 |
| 14 mm | 0.5512 | 35/64 | -0.109 | 9/16 | +0.288 |
| 15 mm | 0.5906 | 19/32 | +0.081 | 9/16 | -0.713 |
| 16 mm | 0.6299 | 5/8 | -0.125 | 5/8 | -0.125 |
| 17 mm | 0.6693 | 43/64 | +0.066 | 11/16 | +0.463 |
| 18 mm | 0.7087 | 45/64 | -0.141 | 11/16 | -0.538 |
| 19 mm | 0.7480 | 3/4 | +0.050 | 3/4 | +0.050 |
| 20 mm | 0.7874 | 25/32 | -0.156 | 13/16 | +0.638 |
| 21 mm | 0.8268 | 53/64 | +0.034 | 13/16 | -0.363 |
| 22 mm | 0.8661 | 55/64 | -0.172 | 7/8 | +0.225 |
| 24 mm | 0.9449 | 15/16 | -0.188 | 15/16 | -0.188 |
| 25 mm | 0.9843 | 63/64 | +0.003 | 1 | +0.400 |
| 27 mm | 1.0630 | 1 1/16 | -0.013 | 1 1/16 | -0.013 |
| 30 mm | 1.1811 | 1 3/16 | +0.163 | 1 3/16 | +0.163 |
| 32 mm | 1.2598 | 1 17/64 | +0.147 | 1 1/4 | -0.250 |
A 64th of an inch is 0.396875 mm, so the nearest 64th is never more than 0.198 mm away. The nearest 16th can be 0.794 mm away, which is too much for a drilled hole and often too much for a wrench.
The mm to inches formula
inches = mm ÷ 25.464ths = inches × 64, rounded to a whole numberdifference in mm = 64ths ÷ 64 × 25.4 - mm
Divide by 25.4 for decimal inches, which is the form a caliper or a machinist uses. For a fraction, multiply by 64 and round to the nearest whole number of 64ths, then reduce. 10 mm gives 0.3937 × 64 = 25.2, so 25/64 in. Converting that fraction back shows the miss: 25/64 in is 9.9219 mm, 0.078 mm under.
What is exact and what is rounded
The 25.4 is exact, because the inch is defined as 25.4 mm. Decimal inches are therefore as good as the digits you keep. The fraction is the rounded part, and the rounding is bounded at half a step: 0.198 mm for 64ths and 0.794 mm for 16ths. The converter reports the actual figure for the size you typed.
Worked examples
A 10 mm bolt head and an inch wrench set
10 ÷ 25.4 = 0.3937 in. The nearest 16th is 3/8 in, which is 9.525 mm: 0.475 mm too small to go on. The next size up in a set that steps by 16ths, 7/16 in, is 11.113 mm and 1.11 mm loose. Neither fits properly, which is the usual result for 10 mm.
A 6 mm hole with fractional bits only
6 ÷ 25.4 = 0.2362 in, and 0.2362 × 64 = 15.12, so the nearest bit is 15/64 in. It drills 5.953 mm, 0.047 mm tight. The 1/4 in bit beside it gives 6.35 mm, 0.35 mm of slop.
19 mm and 3/4 in
19 ÷ 25.4 = 0.748 in, and 0.748 × 64 = 47.87, so the nearest 64th is 48/64, which reduces to 3/4 in. That is 19.05 mm, so a 3/4 in wrench is only 0.05 mm larger than a 19 mm one.
Where mm to inches goes wrong
Taking the nearest fraction for one that fits
Nearest means closest, not large enough. An 8 mm rod is nearest to 5/16 in, which is 7.9375 mm: a hole drilled at that size is 0.0625 mm too small for the rod to pass. For clearance take the next 64th up, 21/64 in, at 8.334 mm.
Rounding small sizes to 16ths
On a 2 mm drill the nearest 16th is 1/16 in, 1.5875 mm, which misses by 21 percent. In 64ths the same size is 5/64 in, 1.984 mm, and the miss drops to 0.016 mm. The smaller the part, the finer the fraction it needs.
Confusing a mil with a millimeter
In US shop talk a mil is a thousandth of an inch, 0.0254 mm, and has nothing to do with millimeters. A 0.5 mm sheet is 19.7 mil thick. Reading "20 mil" as 20 mm makes the material 39 times thicker than it is, and it happens on plastic sheeting and coating specs.
Questions about mm to inches
What is 10 mm in inches as a fraction?
10 mm is 0.3937 in. The nearest 64th is 25/64 in and the nearest 16th is 3/8 in. Neither is exact: 25/64 in is 0.08 mm under and 3/8 in is 0.48 mm under.
Which inch size is closest to 13 mm?
13 mm is 0.5118 in. In 64ths the closest is 33/64 in, 0.097 mm over. The size people usually reach for, 1/2 in, is 12.7 mm, which is 0.3 mm under, so a 13 mm rod will not pass through a 1/2 in hole.
Is 25 mm the same as 1 inch?
No. An inch is 25.4 mm, so 25 mm is 0.9843 in, which rounds to 63/64 in. The 0.4 mm gap is about one sixty-fourth of an inch, visible when two parts are held together.
How accurate is the nearest 1/64 inch?
Within 0.198 mm, which is half of one 64th. That is fine for woodworking and for choosing a drill bit. It is not fine for a bearing fit or a machined part, where the decimal figure in thousandths of an inch is the one to carry.
How do I convert mm to thousandths of an inch?
Divide by 25.4 and move the decimal point three places: 0.2 mm ÷ 25.4 = 0.00787 in, which is 7.9 thousandths. One thousandth of an inch is 0.0254 mm exactly.
Sources
The numbers and rules on this page come from the documents below. Each link opens the original, last checked on October 7, 2026.
- NIST Handbook 44 (2026 edition), Appendix C: General Tables of Units of Measurement gives 1 millimeter as 0.0393701 inch and defines 1 mil as 0.001 inch (exactly), 0.0254 millimeter (exactly).
- NIST Special Publication 811, Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically marks the inch to meter factor 2.54 E-02 as exact, from which 25.4 mm follows.
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