Size of specimens — Cambridge IGCSE Biology
Use magnification = image size ÷ actual size to find magnification and real size, and convert between mm and µm.
Revision notes
1 short note, in the order of the specification. Each one in short:
Magnification and actual size
PracticalMagnification is how many times larger an image is than the actual size of the specimen: magnification = image size ÷ actual size. Measure the image in millimetres and use the same unit for both sizes. Magnification has no units. There are 1000 micrometres in 1 millimetre, so multiply millimetres by 1000 to get micrometres.
Exam questions
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Quick check questions
State the magnification formula.
Show answer
Magnification = image size ÷ actual size.
A cell has an image size of 30 mm and an actual size of 0.1 mm. What is the magnification?
Show answer
× 300.
Supplement How many micrometres are in 1 millimetre?
Show answer
1000 µm.
Supplement Convert 0.5 mm to µm.
Show answer
500 µm.
Frequently asked questions
How do you calculate magnification in biology?
Calculate magnification by dividing image size by actual size: magnification = image size ÷ actual size. Measure the image with a ruler in millimetres, and make sure both sizes are in the same unit. For example, a drawing 36 mm wide of a cell 0.12 mm wide has a magnification of × 300. Magnification has no units.
How do you calculate the actual size of a cell from a photograph?
Rearrange the formula: actual size = image size ÷ magnification. Measure the cell in the photograph in millimetres first. For example, a cell that is 20 mm long in a photograph magnified × 400 has an actual length of 20 ÷ 400 = 0.05 mm. Always give the unit with your answer.
How do you convert mm to µm?
Supplement To convert millimetres to micrometres, multiply by 1000, because there are 1000 µm in 1 mm. To convert micrometres to millimetres, divide by 1000. For example, 0.04 mm is 40 µm, and a nucleus 8 µm wide is 0.008 mm. The micrometre is the unit used for very small things such as cells.
How do you calculate the magnification of a drawing in biology?
Measure the same feature, such as the widest part, on your drawing and on the specimen, both in millimetres. Then divide the drawing length by the specimen length. Both measurements must be in the same unit before you divide, and the answer has no units, so write it as, for example, × 300.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026