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mm to inches Converter

Convert mm to inches and back — exactly 25.4 mm per inch. Free, instant, no sign-up, full working shown.

Convert millimetres ⇄ inches

3.937 in
100 mm = 3.937 in (1 in = 25.4 mm (exact))

Formula: 100 mm ÷ MM_PER_INCH = 3.937 in

How the mm to inches conversion works

The converter applies inches = mm ÷ 25.4. The inch is defined as exactly 25.4 mm under the 1959 international yard and pound agreement, so this conversion is exact — the only rounding is in how many decimal places you display. Going the other way, mm = inches × 25.4.

Worked example: a 100 mm bolt is 100 ÷ 25.4 = 3.937 in, and a 6 in nail is 6 × 25.4 = 152.4 mm. If you work in centimetres instead, use our inches to cm converter.

mm to inches converter questions

How many mm are in an inch?
Exactly 25.4 mm. The inch has been defined as 25.4 mm since the 1959 international yard and pound agreement — it is not an approximation.
How do I convert mm to inches?
Divide by 25.4: inches = mm ÷ 25.4. So 100 mm is 100 ÷ 25.4 = 3.937 in, and 50 mm is 1.9685 in. Type any figure above.
How do I convert inches to mm?
Multiply by 25.4: mm = inches × 25.4. So 4 in is 101.6 mm and 12 in is 304.8 mm. Switch the direction above to do it.
What is 10 mm in inches?
10 mm ÷ 25.4 = 0.3937 in — just under 13/32 of an inch. Common nearby sizes: 5 mm = 0.1969 in, 20 mm = 0.7874 in, 25 mm ≈ 0.9843 in (nearly an inch).
Why do engineering drawings use mm instead of inches?
Millimetres avoid fractions: 3.2 mm is unambiguous, while the same size in inches is 1/8 in plus a rounding error. That is why metric fasteners, drill bits and 3D-printer nozzles are specced in mm worldwide.

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Sources & official guidance

Estimate only — the inch is 25.4 mm by international agreement; conversions are exact. See our disclaimer.