Inches to MM Calculator
Inches to convert
Enter inches as a decimal like 0.75, or a fraction like 3/4.
Millimetres
awaiting inputAwaiting input.
Enter inches to convert.
- Centimetres
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- Metres
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To convert inches to millimetres, multiply by 25.4. 3/4" = 0.75", and 0.75 × 25.4 = 19.05 mm.
Sending Inches into a Metric World
This is the reverse of the mm to inches calculator: instead of bringing a metric spec into inches, this converts an inch measurement so it can be understood by metric drawings, metric hardware and metric-only markets.
A US-made part destined for a metric assembly, a woodworking plan being adapted for a European audience, or a fractional-inch bolt being cross-referenced against a metric parts catalogue — all of these need an inch value converted to millimetres before the comparison makes sense.
The Inches to MM Formula
One multiplication, using an exact constant, does the entire conversion.
MM equals the inch value multiplied by 25.4, the exact number of millimetres in one inch.
- INThe input length in inches — decimal or converted from a fraction.
- 25.4The exact, internationally defined mm-per-inch constant.
- MMThe result: IN × 25.4.
Worked calculation — 3/4"
- 3 ÷ 8 → 3/4 = 0.75Convert the fraction to a decimal (already 0.75)
- 0.75 × 25.4 = 19.05Multiply by the exact constant
3/4" = 19.05 mm
Common Inch Fractions in Millimetres
| Inches | Millimetres |
|---|---|
| 1/8" | 3.175 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 |
| 7/8" | 22.225 mm |
| 1" | 25.4 mm |
| 2" | 50.8 mm |
| 3" | 76.2 mm |
Converting Without a Calculator
- Convert a fraction to a decimal first, if needed. 5/16" = 0.3125".
- Multiply by 25.4. 0.3125 × 25.4 = 7.9375 mm.
Frequently Asked Questions
How do you convert inches to mm?
Multiply the inch value by 25.4. 4 inches × 25.4 = 101.6 mm.
What is 1 inch in mm?
1 inch equals exactly 25.4 millimetres — the internationally defined conversion factor since 1959.
How do you convert a fraction of an inch to mm?
Convert the fraction to a decimal first by dividing the numerator by the denominator, then multiply by 25.4. For 3/8": 3 ÷ 8 = 0.375, and 0.375 × 25.4 = 9.525 mm.
What is 3/4 inch in mm?
3/4" equals 19.05 mm. As a decimal, 3/4" = 0.75", and 0.75 × 25.4 = 19.05.
Why is inches-to-mm conversion always exact?
Because 25.4 is an exact, internationally defined constant, not a rounded approximation. Multiplying any exact inch value by 25.4 produces an exact millimetre result, limited only by how many decimal digits you choose to keep.
MM & Inch Tools
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