Inches to mm: exact since 1959
The international inch is defined as exactly 25.4 mm (international yard and pound agreement, 1959; NIST SP 811). So 1 ft = 304.8 mm, 1 yd = 0.9144 m and 1 mile = 1,609.344 m, all exact. The nautical mile is 1,852 m.
Fractional inch to mm chart
| Inch | Decimal in | mm | cm |
|---|---|---|---|
| 1/16″ | 0.0625 | 1.5875 mm | 0.15875 cm |
| 1/8″ | 0.125 | 3.175 mm | 0.3175 cm |
| 3/16″ | 0.1875 | 4.7625 mm | 0.47625 cm |
| 1/4″ | 0.25 | 6.35 mm | 0.635 cm |
| 5/16″ | 0.3125 | 7.9375 mm | 0.79375 cm |
| 3/8″ | 0.375 | 9.525 mm | 0.9525 cm |
| 1/2″ | 0.5 | 12.7 mm | 1.27 cm |
| 5/8″ | 0.625 | 15.875 mm | 1.5875 cm |
| 3/4″ | 0.75 | 19.05 mm | 1.905 cm |
| 7/8″ | 0.875 | 22.225 mm | 2.2225 cm |
| 1″ | 1 | 25.4 mm | 2.54 cm |
| 1 1/4″ | 1.25 | 31.75 mm | 3.175 cm |
| 1 1/2″ | 1.5 | 38.1 mm | 3.81 cm |
| 2″ | 2 | 50.8 mm | 5.08 cm |
From mm to the nearest fraction
Tools and hardware often come in metric sizes that sit close to an inch fraction. 9.5 mm ÷ 25.4 = 0.374016 in; × 16 = 5.984, which rounds to 6/16 = 3/8″. Choose "Inch fraction" as the target unit to get this automatically, with the error in mm.
Pipe sizes are nominal: a 1/2-inch pipe (NPS 1/2, DN 15) has an outside diameter of 0.840 in (21.3 mm), not 12.7 mm.
Frequently asked questions
How many mm is 1 inch?
Exactly 25.4 mm. Half an inch is 12.7 mm and 3/4 inch is 19.05 mm.
What fraction of an inch is 10 mm?
10 mm = 0.3937 in, which is closest to 13/32″ (0.40625 in) or 3/8″ to the nearest 1/16.
How many meters is 1 foot?
1 ft = 0.3048 m exactly, and 1 m = 3.2808 ft.