One millimetre is 0.03937 inches, and one inch is exactly 25.4 millimetres. The easiest way to hold it in your head is the reverse: 25.4 mm to the inch, so 25 mm is a hair under an inch and 100 mm is just under 4 inches.
Most of the time the awkward part is not the division — it is that the answer needs to come back as a fraction. A tape measure marked in sixteenths does not help you with 0.3937 inches, so the fractional chart below is usually the one people actually need.
| Millimeters | Inches (decimal) | Nearest fraction |
|---|---|---|
| 1 mm | 0.0394 in | 1/16 in |
| 2 mm | 0.0787 in | 5/64 in |
| 3 mm | 0.1181 in | 1/8 in |
| 5 mm | 0.1969 in | 3/16 in |
| 6 mm | 0.2362 in | 1/4 in |
| 10 mm | 0.3937 in | 3/8 in |
| 12 mm | 0.4724 in | 1/2 in |
| 20 mm | 0.7874 in | 13/16 in |
| 25 mm | 0.9843 in | 1 in |
| 25.4 mm | 1.0000 in | 1 in exactly |
| 50 mm | 1.9685 in | 1 15/16 in |
| 100 mm | 3.9370 in | 3 15/16 in |
| 300 mm | 11.8110 in | 11 13/16 in |
| 1000 mm | 39.3701 in | 39 3/8 in |
Fractions are rounded to the nearest sixteenth (or sixty-fourth where that is closer). For machining, work from the decimal — a sixteenth of an inch is 1.5875 mm, which is a large tolerance in metric terms.
| Inches | Millimeters |
|---|---|
| 1/64 in | 0.3969 mm |
| 1/32 in | 0.7938 mm |
| 1/16 in | 1.5875 mm |
| 1/8 in | 3.175 mm |
| 1/4 in | 6.35 mm |
| 3/8 in | 9.525 mm |
| 1/2 in | 12.7 mm |
| 3/4 in | 19.05 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
The 25.4 rule, and why it is exact
Before 1959 the inch was slightly different in different countries — the US inch and the British inch disagreed in the sixth decimal place, which mattered for interchangeable parts. The International Yard and Pound Agreement of 1959 fixed the yard at exactly 0.9144 metres, which makes the inch exactly 25.4 mm and the foot exactly 0.3048 m.
That is why you should never use 0.03937 as a multiplier when precision matters. Divide by 25.4 instead:
- Inches = mm ÷ 25.4
- Millimetres = inches × 25.4
The rounded multiplier is fine for a shelf; it is not fine for a tolerance measured in thousandths.
Reading the answer as a fraction
US tape measures are marked in halves, quarters, eighths, sixteenths and sometimes thirty-seconds. To turn a decimal inch into a fraction, multiply the decimal part by 16 and round:
- 10 mm = 0.3937 in → 0.3937 × 16 = 6.3 → 6/16 = 3/8 in
- 20 mm = 0.7874 in → 0.7874 × 16 = 12.6 → 13/16 in
- 50 mm = 1.9685 in → 0.9685 × 16 = 15.5 → 1 15/16 in
The rounding error is real and worth knowing: 3/8 in is 9.525 mm, so calling 10 mm "three-eighths" is 0.475 mm out. Over a run of ten holes that is nearly 5 mm of accumulated drift, which is how a cabinet ends up out of square.
When it has to fit, mark from the metric measurement directly rather than converting to the nearest fraction.
Where mm and inches collide
Mixed-unit projects are the normal case, not the exception:
- Drill bits and screws: metric bits come in whole millimetres, imperial in fractions. A 6 mm bit (0.2362 in) is slightly smaller than 1/4 in (6.35 mm) — close enough to swap for a rough hole, not close enough for a dowel
- Timber and sheet goods: nominal imperial sizes, metric thicknesses. 18 mm ply is sold where 3/4 in (19.05 mm) used to be, and the 1 mm difference shows in a rebate
- Camera lenses: focal length is always in millimetres. A 50 mm lens is 1.97 in
- Tyres: 225/45R17 mixes a metric width in mm with an imperial rim diameter in inches
- Paper: A4 is 210 × 297 mm, which is 8.27 × 11.69 in — visibly narrower and taller than US Letter at 8.5 × 11 in
- Plumbing and fasteners: pipe and bolt "sizes" are often nominal names rather than real measurements, so convert the measured dimension, not the label
Quick mental conversions
Shortcuts that are close enough to sanity-check a calculation:
- 25 mm ≈ 1 inch (0.6% under)
- 100 mm ≈ 4 inches (1.6% under)
- 1 metre ≈ 39.4 inches, or about 3 ft 3⅜ in
- 300 mm ≈ 1 foot (1.6% under)
- To go from mm to inches fast: divide by 25, then subtract about 1.5%
The "divide by 25" trick is the useful one — it is always slightly generous, so a part cut to that estimate will be a touch long rather than a touch short, which is the direction you want to be wrong in.
Frequently Asked Questions
How many inches is 1 mm?
0.0393701 inches, or roughly 1/25 of an inch. Converting is a division by 25.4, since an inch is defined as exactly 25.4 mm.
How many mm are in an inch?
Exactly 25.4 millimetres. That figure is a definition set by the 1959 International Yard and Pound Agreement, not a measured approximation, so it is exact in both directions.
What is 100 mm in inches?
3.937 inches — just under 4 inches, or about 3 15/16 on a tape measure. 4 inches exactly is 101.6 mm.
Is 10 mm the same as 3/8 inch?
Close but not equal. 10 mm is 0.3937 in and 3/8 in is 0.375 in (9.525 mm), so 10 mm is 0.475 mm larger. Fine for a rough hole, not fine for a press fit or a repeated measurement.
How do I convert mm to a fraction of an inch?
Divide the millimetres by 25.4 to get decimal inches, multiply the decimal part by 16, and round to the nearest whole number for sixteenths. So 20 mm = 0.7874 in, and 0.7874 × 16 = 12.6, giving 13/16 in.
Why not just multiply by 0.03937?
Because it is a rounded value. Dividing by the exact definition of 25.4 avoids compounding a small error across several conversions, which matters for machining tolerances and long runs of repeated measurements.