How do you convert millimeters to inches?
There are exactly 25.4 millimeters in an inch, so divide millimeters by 25.4 to get inches: 100 mm ÷ 25.4 ≈ 3.94 in. For centimeters, divide by 2.54. To go the other way, multiply inches by 25.4. One foot is exactly 304.8 mm, and one metre is about 39.37 inches.
Source: Exact conversion (1 in = 25.4 mm) · Verify against your AHJ and the adopted code edition.
An inch is defined as exactly 25.4 millimeters, so the conversions are clean:
| Convert | Do this |
|---|---|
| mm → inches | ÷ 25.4 |
| cm → inches | ÷ 2.54 |
| m → inches | × 39.37 |
| inches → mm | × 25.4 |
| feet → mm | × 304.8 |
So 100 mm ≈ 3.94 in, 1 metre ≈ 39.37 in (about 3 ft 3⅜ in), and 1 ft = 304.8 mm exactly.
Because the inch is defined by the millimeter, these aren’t rounded approximations — the only rounding is in how many decimals you keep.
Work it out: the feet-inch-fraction converter
parses metric or imperial input (100 mm, 3.94", 1 m) and shows the value in
feet-inches-fraction, decimal, and metric at once — rounding to a buildable
fraction.