Actual sizes of specimens or parts of specimens are often measured in micrometres. You just need to remember that a micrometre is 1000th of a millimetre (or a millionth of a metre).

Lensmagnification calculator

If you need to work out the magnification of a specimen and one figure is in micrometres and the other in millimetres, simply convert one measurement to the unit you want your answer to be in.

A plant cell in the microscope image measures 12mm across. The actual size of the cell is 0.012mm. So, in this instance:

Magnification Calculatormicroscope

So, an actual size of 0.012mm would be 12 micrometres (0.012 X 1000), and an actual size of 0.005mm would be 5 micrometres (0.005 X 1000).