Fraction to Millimeters Calculator
Convert fractional inches (e.g. 3/8 inch) to millimeters (mm) instantly. Essential for engineering, machining, and converting imperial measurements to metric.
Enter Your Fraction
Results update instantly+
inches
=
—
mmDecimal places:
Common Fraction Presets
Reference Table
Click any row to load that fraction into the calculator.
| Fraction | Decimal Inches | mm |
|---|---|---|
| 1/16" | 0.062500" | 1.5875 mm |
| 1/8" | 0.125000" | 3.1750 mm |
| 3/16" | 0.187500" | 4.7625 mm |
| 1/4" | 0.250000" | 6.3500 mm |
| 5/16" | 0.312500" | 7.9375 mm |
| 3/8" | 0.375000" | 9.5250 mm |
| 7/16" | 0.437500" | 11.1125 mm |
| 1/2" | 0.500000" | 12.7000 mm |
| 9/16" | 0.562500" | 14.2875 mm |
| 5/8" | 0.625000" | 15.8750 mm |
| 11/16" | 0.687500" | 17.4625 mm |
| 3/4" | 0.750000" | 19.0500 mm |
| 13/16" | 0.812500" | 20.6375 mm |
| 7/8" | 0.875000" | 22.2250 mm |
| 15/16" | 0.937500" | 23.8125 mm |
| 1" | 1.000000" | 25.4000 mm |
Formula
mm = (whole + numerator ÷ denominator) × 25.4
Example: 3/4 inch = 0.75 × 25.4 = 19.05 mm | 1 inch = 25.4 mm exactly (by definition)
Quick Facts
- 11 inch = 25.4 mm exactly — this is an exact defined conversion, not an approximation.
- 21/16 inch = 1.5875 mm, 1/8 inch = 3.175 mm, 1/4 inch = 6.35 mm.
- 33/4 inch = 19.05 mm — a very common dimension in woodworking and plumbing.
- 4Machinists and engineers frequently need this conversion between imperial drawings and metric tooling.
- 5To reverse: divide mm by 25.4 to get decimal inches, then convert to a fraction.
How to Use
- 1Enter a whole number (optional) — e.g. 1 for 1¾"
- 2Type the numerator (top number) of your fraction
- 3Type the denominator (bottom number)
- 4Result updates instantly — or click a preset
Unit Reference
1 inch25.4 mm
1 inch2.54 cm
1/2 inch0.5000"
1/4 inch0.2500"
1/8 inch0.1250"
1/16 inch0.0625"