1 inch is defined as exactly 25.4 millimeters — which means 1 millimeter equals exactly 1 ÷ 25.4 ≈ 0.03937 inches. Unlike inches-to-centimeters, this conversion never lands on a clean round number in the other direction, which is exactly why a reference table is more useful here than doing the division in your head.
This comes up constantly outside of pure unit conversion homework: matching a metric bolt to an imperial wrench, checking a 3D-printed part's tolerance, or figuring out which drill bit is closest to a millimeter spec on a datasheet. For that last kind of use, the closest common fractional inch matters more than the decimal — so the table below includes both.
MM to Inches, Decimal and Nearest Fraction
| MM | Inches (decimal) | Nearest 1/64″ |
|---|---|---|
| 1 mm | 0.0394 in | 3/64″ |
| 2 mm | 0.0787 in | 5/64″ |
| 3 mm | 0.1181 in | 1/8″ |
| 4 mm | 0.1575 in | 5/32″ |
| 5 mm | 0.1969 in | 13/64″ |
| 6 mm | 0.2362 in | 15/64″ |
| 7 mm | 0.2756 in | 9/32″ |
| 8 mm | 0.3150 in | 5/16″ |
| 9 mm | 0.3543 in | 23/64″ |
| 10 mm | 0.3937 in | 25/64″ |
| 12 mm | 0.4724 in | 15/32″ |
| 15 mm | 0.5906 in | 19/32″ |
| 20 mm | 0.7874 in | 25/32″ |
| 25 mm | 0.9843 in | 63/64″ |
| 25.4 mm | 1.0000 in | 1″ exactly |
| 30 mm | 1.1811 in | 1 3/16″ |
| 40 mm | 1.5748 in | 1 37/64″ |
| 50 mm | 1.9685 in | 1 31/32″ |
| 75 mm | 2.9528 in | 2 61/64″ |
| 100 mm | 3.9370 in | 3 15/16″ |
Fractions are rounded to the nearest 1/64 inch, the finest increment most rulers, calipers, and drill bit sets actually use. At small sizes that rounding is proportionally larger — 1mm's nearest 1/64″ is off by about 0.19mm — so treat the fraction column as a practical match, not a precise one.
Live Calculator
Need a value that isn't on the table? Type into either field — the nearest fraction updates automatically.
Formula: in = mm ÷ 25.4 · mm = in × 25.4
Reading a fraction off a table is fine for a bolt or a drill bit. For anything you need to actually check against a real, physical size on your own screen, Screen Ruler calibrates once and shows you both scales directly.