Screw length
Input: 10 mm
Output: 0.3937 in
10 ÷ 25.4 = 0.3937 inches.
mm to inches converter
Use this for hardware, screw sizes, paper, and screen specs given in millimeters.
Inches:
Formula and algorithm
- inches = millimeters ÷ 25.4
- Constant used: 1 inch = 25.4 millimeters (exact).
- The international inch is identical in the US and UK, so this works for both.
Worked examples
One inch check
Input: 25.4 mm
Output: 1 in
25.4 ÷ 25.4 = exactly 1 inch.
Bolt size
Input: 100 mm
Output: 3.937 in
100 ÷ 25.4 = 3.937 inches.
FAQ
How many mm in an inch?
Exactly 25.4 millimeters make one inch, by international definition.
How do I convert a fraction like 3/8 inch back?
For the reverse, 3/8 inch is 0.375 in × 25.4 = 9.525 mm.
Is this for US or UK inches?
The international inch is identical in both (25.4 mm exactly), so it works everywhere.
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Updated: June 7, 2026