Biology magnification calculator

This calculator uses the magnification formula from biology — magnification = image size ÷ actual size — and rearranges it for you. Enter any two of the three quantities and leave the third blank, and it works out the missing value. It is exactly what you need for microscope questions in GCSE and A-level biology.

The magnification triangle

The three quantities are linked: magnification = image ÷ actual, so image = magnification × actual, and actual = image ÷ magnification. The “image size” is what you measure on the photo or through the microscope; the “actual size” is the specimen’s true size. The crucial rule is that both sizes must be in the same units before you calculate — convert with our length converter if needed (1 mm = 1,000 µm).

A worked example

If a cell measures 40 mm across in a photo and its real size is 0.1 mm (100 µm), the magnification is 40 ÷ 0.1 = ×400. Remember to convert the scale bar or measured length into the same units as the actual size first.

Frequently asked questions

How do I convert mm to µm?

Multiply millimetres by 1,000 to get micrometres (µm). So 0.1 mm = 100 µm.