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How do I convert millimeters to inches?

Short answer

Divide the millimeters by 25.4, because one inch is exactly 25.4 mm. So 10 mm is about 0.3937 inches and 50 mm is about 1.9685 inches. A free converter returns the exact decimal instantly in your browser.

The formula

Inches = millimeters / 25.4. Since the inch is defined as exactly 2.54 cm, which is 25.4 mm, the division is precise. Going the other way, multiply inches by 25.4, which is why a quarter inch is exactly 6.35 mm.

Quick reference for the workshop drawer

Common values: 5 mm is about 0.1969 in, 10 mm is about 0.3937 in, 25.4 mm is exactly 1 in, 50 mm is about 1.9685 in, and 100 mm is about 3.937 in. Wrench sizes, jewelry, camera filters and phone dimensions all bounce between mm and inches, and the converter spares you the long division.

Frequently asked questions

How many millimeters are in an inch?

Exactly 25.4 mm, by definition of the inch.

What is 8 mm in inches?

8 / 25.4 is about 0.315 inches, just under a third of an inch.

What is a quarter inch in mm?

25.4 / 4 = 6.35 mm exactly.

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