The conversion table
| Fraction | Decimal inch | Millimetres |
|---|---|---|
| ⅛ in | 0.125 | 3.175 |
| ¼ in | 0.25 | 6.35 |
| ⅜ in | 0.375 | 9.525 |
| ½ in | 0.5 | 12.7 |
| ⅝ in | 0.625 | 15.875 |
| ¾ in | 0.75 | 19.05 |
| ⅞ in | 0.875 | 22.225 |
| 1 in | 1 | 25.4 |
How to work out any fraction
Divide the top number by the bottom, then multiply by 25.4. For ⅝ in: 5 ÷ 8 = 0.625, and 0.625 × 25.4 = 15.875 mm.
Rulers and tape measures usually show whole millimetres, so 15.875 mm is read as just under 16 mm. A ¼ inch seam allowance is 6.35 mm, a little more than 6 mm.