Inches to Millimeters Converter
1 inch (in) equals 25.4 millimeters (mm).
Quick answer
To convert inches to millimeters, multiply by 25.4 (exact). A quarter inch is 6.35 mm, a half inch 12.7 mm.
- 1 inch = 25.4 millimeters
- 1 millimeter = 0.03937007874 inches
- Formula: mm = in × 25.4
- Quick estimate: multiply by 25 and add a little: 3 in × 25 = 75, exact 76.2 mm.
1 inch = 25.4 millimeters
How to convert inches to millimeters
mm = in × 25.4
To convert inches to millimeters, multiply the number of inches by 25.4.
Example: 42 in to mm
- Start with the value in inches: 42 in.
- Multiply by 25.4: 42 × 25.4 = 1,066.8.
- Result: 42 inches = 1,066.8 millimeters.
Going the other way, 1 millimeter = 0.03937007874 inches, so to convert millimeters back to inches you divide by the same number. See millimeters to inches.
Where the conversion factor comes from
Since the international yard and pound agreement of 1959, the inch has been defined as exactly 2.54 centimeters. Before that, the US and the UK used slightly different inches (the US inch was 2.540005 cm). The 1959 definition made the conversion exact, so 2.54 is not a rounded number: every inch-to-centimeter conversion is a multiplication by exactly 2.54. So 1 inch = 25.4 mm exactly.
Mental math: in to mm in your head
Multiply by 25 (multiply by 100, divide by 4), then add 1.6%: 8 in → 800 ÷ 4 = 200, plus a little = 203 mm (exact: 203.2).
Fractions to memorize: ⅛ in = 3.2 mm, ¼ in = 6.35 mm, ⅜ in = 9.5 mm, ½ in = 12.7 mm, ¾ in = 19.05 mm.
When you need to convert inches to millimeters
- Woodworking and metalworking with US plans and a metric ruler.
- Pipe, hose and fitting sizes.
- Tire and wheel rim sizes (rims are in inches, tire widths in mm).
- Screen and sensor sizes.
Common mistakes
- Assuming pipe sizes are actual dimensions: a ½-inch pipe (nominal) has an outside diameter of about 21.3 mm.
- Rounding ¼ in to 6 mm for precision work: the difference (0.35 mm) matters for fits.
- Confusing 1/16 in (1.59 mm) with 1.6 mm drill bits in tight tolerances.
Everyday inches in millimeters
| Example | Inches | Millimeters |
|---|---|---|
| 1/16 inch | 0.0625 in | 1.5875 mm |
| 1/8 inch | 0.125 in | 3.175 mm |
| 1/4 inch (common drill bit) | 0.25 in | 6.35 mm |
| 1/2 inch | 0.5 in | 12.7 mm |
| 3/4 inch plywood (nominal) | 0.75 in | 19.05 mm |
| 2 inches | 2 in | 50.8 mm |
Inches to millimeters conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.1 in | 2.54 mm |
| 0.2 in | 5.08 mm |
| 0.3 in | 7.62 mm |
| 0.4 in | 10.16 mm |
| 0.5 in | 12.7 mm |
| 0.6 in | 15.24 mm |
| 0.7 in | 17.78 mm |
| 0.8 in | 20.32 mm |
| 0.9 in | 22.86 mm |
| 1 in | 25.4 mm |
| 1.5 in | 38.1 mm |
| 2 in | 50.8 mm |
| 2.5 in | 63.5 mm |
| 3 in | 76.2 mm |
| 3.5 in | 88.9 mm |
| 4 in | 101.6 mm |
| 4.5 in | 114.3 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 7 in | 177.8 mm |
| 7.5 in | 190.5 mm |
| 8 in | 203.2 mm |
| 9 in | 228.6 mm |
| 10 in | 254 mm |
| 11 in | 279.4 mm |
| 12 in | 304.8 mm |
| 13 in | 330.2 mm |
| 14 in | 355.6 mm |
| 15 in | 381 mm |
| 16 in | 406.4 mm |
| 17 in | 431.8 mm |
| 18 in | 457.2 mm |
| 19 in | 482.6 mm |
| 20 in | 508 mm |
| 21 in | 533.4 mm |
| 22 in | 558.8 mm |
| 23 in | 584.2 mm |
| 24 in | 609.6 mm |
| 25 in | 635 mm |
| 26 in | 660.4 mm |
More in to mm values
- 27 in
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What is an inch?
The inch is a unit of length in the imperial and US customary systems equal to exactly 2.54 centimeters.
What is a millimeter?
The millimeter (also spelled millimetre) is a metric unit of length equal to one thousandth of a meter, or one tenth of a centimeter.
Convert inches to other units
Convert other units to millimeters
Frequently asked questions
How many millimeters are in an inch?
One inch is 25.4 millimeters.
How many inches are in a millimeter?
One millimeter is 0.03937007874 inches.
What is the formula to convert inches to millimeters?
mm = in × 25.4.
What is 1/4 inch in mm?
1/4 inch is exactly 6.35 mm.
What is 3/8 inch in mm?
3/8 inch is 9.525 mm.