1 mm = 1000 μm
1 μm = 1000 nm
1 μm = 1 × 10⁻⁶ m, 1 nm = 1 × 10⁻⁹ m

Cells are tens of micrometres across; ribosomes are about 20 nm. Very small sizes are written in standard form.

magnification = size of image ÷ size of real object

1. A cell drawn 24 mm long is really 30 μm long
2. 24 mm = 24 000 μm
3. magnification = 24 000 ÷ 30 = × 800

Put both sizes in the same unit before you divide. Total magnification of a light microscope = eyepiece × objective.