Definition
Millimetre
A millimetre is a metric unit for measuring very small lengths.
What it means
Millimetres are useful for short lengths or thin objects when centimetres are too large. On a metric ruler, the smallest numbered-centimetre gap is usually split into ten millimetres.
Remember it
Ten millimetres fit into one centimetre; 1,000 millimetres fit into one metre.
Example
A strip is 24 mm long and another strip is 16 mm long. Together they measure 24 + 16 = 40 mm, which is 4 cm.
Watch for
A millimetre measures length. Square millimetres measure area, and cubic millimetres measure volume; those are different units.
Common mix-up
mm means millimetre; mL means millilitre. Their letters look similar, but they measure different quantities.
Exact meaning
A millimetre, symbol mm, is one-thousandth of a metre. Exactly 1,000 mm = 1 m, and 10 mm = 1 cm.