Cells and the structures inside them are far too small to measure sensibly in metres, so biologists use smaller units. Each unit in the list below is 1000 times smaller than the one before it.
- millimetre (mm): milli means 10−3, one thousandth of a metre
- micrometre (µm): micro means 10−6, one millionth of a metre
- nanometre (nm): nano means 10−9, one billionth of a metre
- picometre (pm): pico means 10−12, one trillionth of a metre
To change to a smaller unit (mm to µm, or µm to nm), multiply by 1000. To change to a bigger unit, divide by 1000. For example, 0.02 mm × 1000 = 20 µm, which is a typical size for an animal cell. In the same way 3 mm is 3000 µm, and 25 nm ÷ 1000 = 0.025 µm, because a micrometre is a bigger unit than a nanometre.
Very small lengths are neatly written in standard form, as a number between 1 and 10 multiplied by a power of ten. A bacterium 2 µm long is 2 × 10−6 m.