Length conversion
Convert Inches to Millimeters
1 inch equals exactly 25.4 millimeters. To convert inches to millimeters, multiply the number of inches by 25.4.
Inches to millimeters is the conversion for hardware, machining, 3D-printing and woodworking when a spec is imperial but your tools or CAD work in mm. Enter any inch value for an instant millimeter result with the exact factor shown.
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Computed at full precision in your browser; the result is rounded for display.
How to convert inches to millimeters
Formula
millimeters = inches × 25.4 inches = millimeters ÷ 25.4One inch is exactly 25.4 millimeters by the 1959 international agreement, so this conversion is exact — useful when matching imperial parts to metric tooling.
| Inches | Millimeters |
|---|---|
| 0.125 in | 3.175 mm 1/8 inch |
| 0.25 in | 6.35 mm 1/4 inch |
| 0.5 in | 12.7 mm 1/2 inch |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 6 in | 152.40 mm |
| 12 in | 304.80 mm 1 foot |
| 24 in | 609.60 mm 2 feet |
| 36 in | 914.40 mm 1 yard |
Worked example
A drawing calls for a 1/4-inch hole and your drill set is metric. Multiply 0.25 × 25.4 to get 6.35 mm — the nearest metric bit is 6.5 mm, slightly larger. A 2-inch board is exactly 50.8 mm, and a 1-inch dowel is 25.4 mm.
Shortcut. Rule of thumb: an inch is about 25 mm, so multiply by 25 for a rough figure and nudge up 1.6%. Fractions: 1/8 in ≈ 3.2 mm, 1/4 in ≈ 6.4 mm, 1/2 in ≈ 12.7 mm.
Good to know
Inches-to-millimeters is a straight multiply-by-25.4 conversion. Convert fractional inches to decimals first (1/8 = 0.125, 3/8 = 0.375). Metric and imperial fasteners are rarely interchangeable even when the millimeter value looks close, so check tolerances before swapping parts.
Frequently asked questions
How many mm is 1 inch?
Exactly 25.4 millimeters. Multiply any number of inches by 25.4 to get millimeters.
What is 1/4 inch in mm?
A quarter inch is 6.35 mm. An eighth inch is 3.175 mm and a half inch is 12.7 mm.
How do I convert inches to mm in my head?
Multiply by 25 for a quick estimate, then add about 1.6%. A 4-inch pipe is roughly 100 mm (101.6 mm exactly).
Is 1 inch exactly 25.4 mm?
Yes. Since 1959 the inch is defined as exactly 25.4 millimeters, so the conversion has no rounding error.
What is 12 inches in mm?
12 inches — one foot — is exactly 304.8 mm. A yard (36 inches) is 914.4 mm.