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Italiano1 inch = 25.4 mm: the formula
To convert inches to mm, multiply by 25.4: mm = inches × 25.4. The factor is exact. Example: an 18 in wheel is 18 × 25.4 = 457.2 mm across.
The converter on this page is already set to inches to millimeters and opens on 1 in = 25.4 mm. Results update as you type, to six significant figures. For a fraction, enter the decimal: 0.375 for 3/8 gives 9.525 mm. The ⇄ "Swap units" button turns it into mm to inches, and the "In every unit" table adds cm, feet and meters.
Fractions of an inch in mm
| Inches | Decimal | Millimeters | Where you see it |
|---|---|---|---|
| 1/16 | 0.0625 | 1.5875 | Common tick on a tape measure |
| 1/8 | 0.125 | 3.175 | Small drill bit |
| 3/16 | 0.1875 | 4.7625 | Drill bit, close to 5 mm |
| 1/4 | 0.25 | 6.35 | Hex driver bits, smallest ratchet drive |
| 5/16 | 0.3125 | 7.9375 | Bolt, close to 8 mm |
| 3/8 | 0.375 | 9.525 | Mid-size ratchet drive |
| 1/2 | 0.5 | 12.7 | Large ratchet drive |
| 9/16 | 0.5625 | 14.2875 | Wrench, close to 14 mm |
| 3/4 | 0.75 | 19.05 | Plywood sheet, 19 mm socket |
| 1 | 1 | 25.4 | One inch |
| 1 1/2 | 1.5 | 38.1 | Actual thickness of a 2×4 |
| 3 1/2 | 3.5 | 88.9 | Actual width of a 2×4 |
| 18 | 18 | 457.2 | Car wheel |
In the shop and the garage
Wrenches and sockets
A 3/4 in socket is 19.05 mm and usually fits a 19 mm nut. A 9/16 in wrench is 14.2875 mm, loose on a 14 mm nut, and a 1/2 in wrench at 12.7 mm will not go over a 13 mm one. Ratchet drives of 1/4, 3/8 and 1/2 in are square drive sizes of 6.35, 9.525 and 12.7 mm, not nut sizes.
Pipe sizes are names, not measurements
A 1/2 in pipe is not 12.7 mm across. US 1/2 in pipe has an outside diameter of 0.840 in, or 21.336 mm, and British G 1/2 threads measure about 20.955 mm. The inch figure is a trade size inherited from old bore sizes: match fittings by that name, not by converting it.
Lumber and bike tires
A 2×4 is really 1 1/2 × 3 1/2 in, which is 38.1 × 88.9 mm. On a 29 × 2.4 in mountain bike tire, the 2.4 in width is 60.96 mm, while the rim itself is marked ISO 622 mm, the same as a 700c road wheel.
Frequently asked questions
What is 5/16 in in mm?
7.9375 mm. Divide 5 by 16 to get 0.3125, then multiply by 25.4. An 8 mm bit is the closest metric size.
How many mm is 1 1/4 inches?
31.75 mm. Type 1.25 into the converter to get it.
Why is the factor exactly 25.4?
In 1959 the US, the UK and other Commonwealth countries agreed to define the inch as exactly 25.4 mm. Before that, the US and British inches differed by a few millionths of an inch.