Cm to inches converter

The rule

One inch is exactly 2.54 centimeters. This has been the definition since 1959 (NIST Handbook 44, Appendix C), so the conversion is exact both ways. Centimeters to inches: divide by 2.54. Inches to centimeters: multiply by 2.54.

Worked example

30 cm ÷ 2.54 = 11.811 inches. On a tape measure marked in sixteenths, 0.811 of an inch is closest to 13/16, so look for 11 13/16 in. Going back, 11.811 × 2.54 = 30 cm.

Reading a tape measure

US tape measures and rulers mark inches in halves, quarters, eighths and sixteenths. The answer card shows the nearest sixteenth, which is within 1/32 inch (0.8 mm) of the exact figure. You can also type fractions in the inches box: 5 1/2 or 3/4 both work.

A quick check in your head

An inch is a little over 2.5 cm, so divide centimeters by 2.5 and take a bit off: 30 cm is about 12, and exactly 11.8. Ten centimeters is just under 4 inches. For heights in feet and inches, use cm to feet; for small sizes such as screws, use mm to inches.

Rounding

The answer is worked out from the exact definition, then shown to 4 decimal places (6 significant figures for numbers below 1). It says "exactly" only when nothing was rounded.