Mm to cm converter

The rule

There are exactly 10 millimeters in a centimeter, because "milli" means one thousandth of a meter and "centi" one hundredth (SI Brochure, International Bureau of Weights and Measures). So divide millimeters by 10 to get centimeters, and multiply centimeters by 10 to get millimeters. Nothing is ever rounded in this direction except to tidy very long decimals.

Worked example

25 mm ÷ 10 = 2.5 cm. Going back, 2.5 cm × 10 = 25 mm. In practice you move the decimal point one place: left for mm to cm, right for cm to mm. 7 mm is 0.7 cm and 0.5 cm is 5 mm.

On a ruler

A metric ruler has numbered centimeter marks and ten small millimeter marks between each one. Count whole centimeters to the last number, then the small marks: 2 numbered marks and 5 small ones is 2.5 cm, or 25 mm.

Inches as well

The answer card also shows the size in inches, since 25.4 mm is exactly 1 inch, and the nearest sixteenth of an inch for a US tape measure. For many small sizes at once, use mm to inches. Engineers and builders usually work in millimeters only, so a drawing marked 1200 means 1200 mm, which is 120 cm or 1.2 m.

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.