Inches to Millimeters (in to mm)
1 in = 25.4 mm
To convert inches to millimeters: millimeters = inches × 25.4
| inches | millimeters |
|---|---|
| 0.0625 in | 1.587 mm |
| 0.125 in | 3.175 mm |
| 0.25 in | 6.35 mm |
| 0.375 in | 9.525 mm |
| 0.5 in | 12.7 mm |
| 0.625 in | 15.88 mm |
| 0.75 in | 19.05 mm |
| 1 in | 25.4 mm |
Multiply inches by 25.4 to get millimeters. For fractional inches, convert the fraction to a decimal first: 3/8 in = 0.375 in = 9.525 mm.
Formula
millimeters = inches × 25.4
Example: 0.375 inches = 9.525 millimeters.
How to convert inches to millimeters in your head
- Multiply by 25 and add 2%: 4 in × 25 = 100, plus 2 = 102 mm (exact: 101.6).
- Anchor on the quarter inch: 1/4 in is 6.35 mm, so 1/2 in is 12.7 mm and 3/4 in is 19.05 mm.
- Each 1/16 in is about 1.6 mm (exact 1.5875), so 5/16 in ≈ 5 × 1.6 = 8 mm (exact 7.94).
Inches to Millimeters chart
| inches | millimeters |
|---|---|
| 0.0625 | 1.5875 |
| 0.125 | 3.175 |
| 0.25 | 6.35 |
| 0.375 | 9.525 |
| 0.5 | 12.7 |
| 0.625 | 15.875 |
| 0.75 | 19.05 |
| 1 | 25.4 |
| 1.5 | 38.1 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 6 | 152.4 |
| 12 | 304.8 |
Inch fractions to millimeters for drill bits, sockets and lumber
Fractions are converted exactly (1/16 in = 0.0625 in). Values are rounded for display, not for machining.
| What | inches | millimeters | Centimeters |
|---|---|---|---|
| 1/16 in | 0.0625 | 1.5875 | 0.15875 |
| 1/8 in | 0.125 | 3.175 | 0.3175 |
| 3/16 in | 0.1875 | 4.7625 | 0.47625 |
| 1/4 in | 0.25 | 6.35 | 0.635 |
| 5/16 in | 0.3125 | 7.9375 | 0.79375 |
| 3/8 in | 0.375 | 9.525 | 0.9525 |
| 1/2 in | 0.5 | 12.7 | 1.27 |
| 5/8 in | 0.625 | 15.875 | 1.5875 |
| 3/4 in | 0.75 | 19.05 | 1.905 |
| 1 in | 1 | 25.4 | 2.54 |
| Nominal 2x4 actual thickness (1½ in) | 1.5 | 38.1 | 3.81 |
| 2 | 50.8 | 5.08 | |
| Nominal 2x4 actual width (3½ in) | 3.5 | 88.9 | 8.89 |
| 6 | 152.4 | 15.24 | |
| 1 foot | 12 | 304.8 | 30.48 |
| 1 yard | 36 | 914.4 | 91.44 |
Good to know
Common fractions: 1/16 in = 1.5875 mm, 1/8 = 3.175 mm, 1/4 = 6.35 mm, 3/8 = 9.525 mm, 1/2 = 12.7 mm, 5/8 = 15.875 mm, 3/4 = 19.05 mm.
Pipe sizes are an exception: a '1/2 inch' pipe is a nominal size, and its actual outside diameter is larger. Check the manufacturer's table for plumbing parts.
Read next: Fractional inches to mm: drill bit, wrench and socket size chart
Common inches to millimeters mistakes
- Typing 3/8 as 3.8. Convert the fraction to a decimal first: 3 ÷ 8 = 0.375 in = 9.525 mm.
- Using 25 instead of 25.4 for big numbers. At 36 in the shortcut is 900 mm; the real value is 914.4 mm.
- Reading nominal lumber and pipe sizes as real sizes. A 2x4 is 1½ × 3½ in (38.1 × 88.9 mm), and a 1/2 in pipe's outside diameter is about 0.84 in.
- Assuming metric and inch tools swap at every size. 1/4 in (6.35 mm) is close to 6 mm, but 3/4 in (19.05 mm) and 19 mm differ by 0.05 mm, and 5/8 in (15.875 mm) sits between 15 and 16 mm. The inch is exactly 25.4 mm (NIST, 1959 agreement).
Sources
Frequently asked questions
How many mm is 1 inch?
Exactly 25.4 mm.
How many mm is 3/8 inch?
3/8 in is 9.525 mm.
How many mm is 1/2 inch?
1/2 in is 12.7 mm.
How many mm is 5/16 inch?
5/16 in is 7.9375 mm, about 7.94 mm.
How many mm is 3/4 inch?
3/4 in is 19.05 mm.
How many mm is 1/8 inch?
1/8 in is 3.175 mm.
How many mm is 2 inches?
2 in is 50.8 mm.
How many mm is 6 inches?
6 in is 152.4 mm.
How many mm is 12 inches?
12 in (one foot) is 304.8 mm.
How many mm is 0.1 inch?
0.1 in is 2.54 mm. A thou (0.001 in) is 0.0254 mm.