Magnification calculator for biology

Cells are usually measured in micrometres (µm): 1 µm is a thousandth of a millimetre. Viruses and cell structures under an electron microscope are measured in nanometres (nm).

• Magnification: a drawing of a cell is 20 mm across and the real cell is 0.1 mm. $20 \div 0.1 = \times 200$.
• Real size: a cell's image measures 50 mm at ×400. Real size $= 50 \div 400 = 0.125$ mm $= 125$ µm.
• Image size: a 10 µm cell at ×1,500. Image $= 10 \times 1500 = 15000$ µm $= 15$ mm.

Put image on top and actual × magnification underneath:

• image = actual × magnification
• actual = image ÷ magnification
• magnification = image ÷ actual

Measure the image with a ruler in mm, then convert if the question wants µm.

A light microscope's total magnification is the eyepiece lens × the objective lens (for example ×10 × ×40 = ×400). Electron microscopes have much higher magnification and resolution (the ability to tell two close points apart), so they show sub-cellular structures like ribosomes and the inside of mitochondria.

• Dividing a size in mm by a size in µm.
• Putting the real size on top.
• Adding units to the magnification.

Practise with our magnification questions and our guide to GCSE biology topics.

• Measure the image with a ruler in mm, then convert to µm if the real size is in µm.
• Use a scale bar if one is given: magnification = length of scale bar measured ÷ length it represents.
• Give real sizes in sensible units: 0.000025 m is easier to read as 25 µm.
• Round magnifications sensibly (×200, not ×200.4).