What this calculator does
Converting mm to imperial means turning a metric measurement into inches, and in trade and engineering work that usually means a fraction of an inch rather than a long decimal, because drill bits, spanners and fasteners are sized in fractions such as 1/4 in or 5/16 in, not in 0.3125 in.
This calculator does both steps at once: it gives the exact decimal inch figure, then rounds that to the nearest sixty-fourth of an inch, which is the resolution most fractional rulers, tap and drill charts and imperial socket sets use. It also works the other way, turning a fraction or decimal inch measurement back into millimetres.
The formula
Divide the millimetre value by 25.4 to get inches, since 1 inch is defined as exactly 25.4 mm. To express that as a fraction, multiply the decimal inches by 64, round to the nearest whole number, then simplify that fraction of 64ths (for example 32/64 simplifies to 1/2). Converting the other way multiplies inches by 25.4 to get millimetres.
| Term | Meaning |
|---|---|
| mm | Millimetre, one-thousandth of a metre, the standard small metric length unit. |
| Decimal inch | The inch value as a plain decimal, such as 0.3937 in. |
| Nearest 1/64 fraction | That decimal inch rounded to the closest sixty-fourth, the resolution used on most fractional rulers and imperial drill/tap charts. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The value to convert: millimetres if converting to imperial, or inches (as a decimal) if converting to millimetres. |
| Convert | Choose which direction you are converting. |
When to use it
Reading a metric drawing with imperial tools
A drawing dimensioned in millimetres still needs to be cut, drilled or measured with an imperial tape or drill set in many workshops, so each key dimension gets converted to the nearest usable fraction.
Matching a drill bit or fastener size
Drill bit and fastener charts are usually printed in one system or the other. Converting a metric bolt diameter to its nearest imperial fraction shows which standard drill or spanner size is closest.
Checking a measurement across two unit systems
When a supplier quotes a part in millimetres and a spec sheet lists it in inches, converting one to the other confirms whether they actually match before ordering.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Mm to imperial chart: common millimetre values
A quick-reference chart for common small measurements, from 1 mm up to 25.4 mm (1 inch).
| Millimetres | Nearest imperial fraction | Inches (decimal) |
|---|---|---|
| 1 mm | 3/64 in | 0.0394 in |
| 2 mm | 5/64 in | 0.0787 in |
| 3 mm | 1/8 in | 0.1181 in |
| 5 mm | 13/64 in | 0.1969 in |
| 6 mm | 15/64 in | 0.2362 in |
| 10 mm | 25/64 in | 0.3937 in |
| 12.7 mm | 1/2 in | 0.5000 in |
| 19 mm | 3/4 in | 0.7480 in |
| 25.4 mm | 1 in | 1.000 in |
Questions
Why round to the nearest 1/64 inch and not something coarser?
A sixty-fourth of an inch is the finest graduation printed on most fractional machinist rulers and is the standard resolution for imperial drill and tap size charts, so it matches the tools most people are converting for.
Is 1 inch exactly 25.4 mm?
Yes. The inch was redefined in 1959 by international agreement to be exactly 25.4 mm, so this conversion carries no rounding error until the final step of expressing it as a fraction.
Why does my fraction not match a fastener size exactly?
Metric and imperial fastener series were designed independently, so a metric bolt diameter rarely lands exactly on a standard imperial size. The nearest fraction shown is a rounding, not a guarantee that an imperial part of that size will fit.
What if I need more precision than 1/64 inch?
Use the decimal inches figure directly rather than the fraction. The decimal conversion (mm ÷ 25.4) is exact to as many places as you need; the fraction is only a convenience for reading off a ruler or chart.
For a general inches-to-centimetres conversion without the fractional rounding, see the inches to cm calculator.