How to Answer "Calculate" Questions in IGCSE Biology
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Calculate means show the formula, substitute the numbers, give the answer with correct units. Always show working: method marks are available even if the final answer is wrong.
"Calculate" means work out a numerical answer using the data given. You must show your working: a correct answer without working may lose method marks. Always include units in your final answer.
The three rules
1. Write the formula: even if it seems obvious, writing the formula earns a method mark if your final answer is wrong.
2. Show substitution: show the numbers going into the formula before giving the answer.
3. Include units: a number without units may lose the final mark even if the calculation is correct.
Key IGCSE Biology calculations
Magnification: magnification = image size ÷ actual size (rearrange as needed)
Population estimate: mean per quadrat × (total area ÷ quadrat area)
Genetic ratios: from Punnett squares (e.g. 3:1, 1:1)
Unit conversions to know
1 mm = 1000 μm. 1 cm = 10 mm. 1 m = 1000 mm.
The most common conversion error is between mm and μm in magnification questions. Always check that both values are in the same unit before dividing.
Frequently asked questions
Do I get marks for working even if the final answer is wrong?
Yes. If you write the correct formula and substitute correctly, you can earn 1–2 method marks even with an arithmetic error. Never leave a calculate question blank.
How many decimal places should I give?
Match the precision of the data given. If the data has one decimal place, give your answer to one decimal place. If unsure, two decimal places is usually appropriate.