Magnification and actual sizeSpec B2.2.1, B2.2.2, B2.2.3
In short
Magnification 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.
Many biological specimens are too small to see clearly. A microscope makes an image that is larger than the actual size of the specimen. The magnification tells you how many times larger.
Rearranged: actual size = image size ÷ magnification, and image size = actual size × magnification. Measure the image with a ruler, in millimetres. Magnification has no units.
Calculating magnification
A drawing of a cell is 36 mm wide. The actual width of the cell is 0.12 mm. Calculate the magnification.
- Both sizes are in mm, so no conversion is needed.
- magnification = image size ÷ actual size = 36 ÷ 0.12 = 300.
Answer: × 300
Calculating actual size
A photograph of a plant cell is magnified × 400. The cell in the photograph is 20 mm long. Calculate the actual length of the cell in mm.
- Rearrange: actual size = image size ÷ magnification.
- actual size = 20 ÷ 400 = 0.05 mm.
Answer: 0.05 mm
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanConverting between mm and µm
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanThe unit for very small things is the micrometre (µm). There are 1000 µm in 1 mm. To change mm to µm, multiply by 1000. To change µm to mm, divide by 1000.
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanConverting units
A cell has an actual width of 0.04 mm. Give this in µm. A nucleus is 8 µm wide. Give this in mm.
- mm to µm: 0.04 × 1000 = 40 µm.
- µm to mm: 8 ÷ 1000 = 0.008 mm.
Answer: 40 µm; 0.008 mm
Image size and actual size must be in the same unit (mm) before you divide. Magnification has no units: write × 300, not 300 mm.
To find the magnification of your drawing, measure the same feature (e.g. the widest part) on the drawing and on the specimen, both in mm, then divide drawing length by specimen length. Draw a clear, continuous outline with no shading, and label with ruled lines.
Quick check
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.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026