FractionFractional inches to millimetres
Type a fraction of an inch, get millimetres — exactly, because the inch is defined as 25.4 mm. For the reverse question, millimetres to the nearest usable fraction, use the metric matcher.
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Common fractional inches in millimetres
| Inches | Millimetres |
|---|---|
| 1/16″ | 1.5875 mm |
| 1/8″ | 3.175 mm |
| 3/16″ | 4.7625 mm |
| 1/4″ | 6.35 mm |
| 3/8″ | 9.525 mm |
| 1/2″ | 12.7 mm |
| 5/8″ | 15.875 mm |
| 3/4″ | 19.05 mm |
| 1″ | 25.4 mm |
The full rule, 1/64″ through 1″, is on the fraction–metric chart, printable.
Common questions
- How many millimetres is a fractional inch?
- Multiply by 25.4, which is exact: one inch has been defined as exactly 25.4 mm since 1959. So 3/8″ is 3/8 × 25.4 = 9.525 mm, with no rounding involved.
- Is 25.4 mm per inch an approximation?
- No. The international yard and pound agreement of 1959 defines the inch as exactly 25.4 mm, so every conversion on this page is exact arithmetic, not an estimate.
- How do I go the other way, from millimetres to a fraction?
- Use the metric-to-fraction matcher, which finds the nearest fractional inches for any millimetre value and shows the exact error of each candidate.