mm to inches and inches to mm converter
mm to inches, instantly: 1 inch is exactly 25.4 mm, so 25.4 mm = 1 in. Type millimeters and the inches appear while you type, together with the nearest fraction a tape measure shows (for example 8 mm = 0.315 in, about 5/16 in). The reverse, inches to mm, works in the same box.
Example: Length 25.4 · Direction: mm to inches (mm → in) → Answer: 25.4 mm = 1 in. Source: NIST SP 811, Appendix B.8 – inch = 25.4 mm (exact, international agreement of 1959). Updated: .
How it is calculated
How to convert mm to inches
inches = millimeters ÷ 25.4. For 100 mm: 100 ÷ 25.4 = 3.937 in. Going back, millimeters = inches × 25.4: 0.5 in × 25.4 = 12.7 mm. The number 25.4 is exact, not a rounded value: since the international agreement of 1959 one inch is defined as 25.4 millimeters. One millimeter is 0.03937 inch.
From a decimal to the fraction on the tape
Tape measures, drill bits and spanners in the US use fractions: 1/16, 1/8, 1/4 and so on, down to 1/64. The converter therefore also gives the nearest 1/64 fraction and tells you how far it is off. Take 8 mm: 8 ÷ 25.4 = 0.315 in, and 20/64 = 5/16 in (0.3125 in = 7.94 mm), about 0.06 mm short. For 12.7 mm the answer is exactly 1/2 in, with no deviation.
Do not swap metric and inch tools
Neighbouring sizes are close but not equal: 3/8 in is 9.525 mm, while a 10 mm socket is 0.475 mm bigger and can round off a 3/8 in hex head. 1/2 in is 12.7 mm against a 13 mm spanner (0.3 mm of play). Check the chart below before you reach for the nearest tool.
Thousandths of an inch
In machining, 0.001 in is one “thou” or mil, and it equals 0.0254 mm. A tolerance of 0.1 mm is therefore 3.937 thou, and a 0.76 mm bank card is 0.0299 in (about 30 thou).
A quick mental trick
Divide the millimeters by 25 (or by 25.4 if you have a calculator): 50 mm gives 2 in against 1.969 in exact. For small jobs the 1.6 % error is acceptable; for fits and tolerances use the exact result above.
For many units at once see the length converter; for centimeters see cm to inches.
Frequently asked questions
How many inches is 1 mm?
1 millimeter is 0.03937 inch (1 ÷ 25.4). 10 mm is 0.3937 in and 100 mm is 3.937 in.
How many mm are in an inch?
Exactly 25.4 mm. Half an inch is 12.7 mm, a quarter inch 6.35 mm and an eighth of an inch 3.175 mm.
What is 8 mm in inches as a fraction?
8 mm = 0.315 in, and the nearest fraction is 5/16 in (0.3125 in = 7.94 mm). It is not an exact match, the difference is about 0.06 mm.
How do I convert inches to mm?
Multiply by 25.4. Example: 3 in × 25.4 = 76.2 mm. For fractions, turn them into decimals first: 3/8 = 0.375 in, and 0.375 × 25.4 = 9.525 mm.
How do I convert mm to inches in my head?
Divide by 25. 75 mm ÷ 25 = 3 in (exact: 2.953 in). The result is about 1.6 % too high.
Sources and legal basis
- NIST SP 811, Appendix B.8 – inch = 25.4 mm (exact, international agreement of 1959)
- BIPM – The International System of Units (SI Brochure)
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