How to Calculate Percentage Change 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
Percentage change = ((final − initial) ÷ initial) × 100. Always divide by the INITIAL value. Show the formula, substitution, and answer. Include the sign (+ or −) to show direction of change.
Percentage change calculations appear in osmosis experiments, growth investigations, population studies, and many other IGCSE Biology contexts. The formula is always the same, but students frequently lose marks through arithmetic errors or forgetting to show working.
If the result is positive, the value increased. If negative, it decreased. Always divide by the initial value, not the final value.
Worked examples
Example 1: A potato cylinder has an initial mass of 2.5 g and a final mass of 3.0 g. % change = ((3.0 − 2.5) ÷ 2.5) × 100 = (0.5 ÷ 2.5) × 100 = +20% (mass increased by 20%)
Example 2: A potato cylinder has an initial mass of 3.2 g and a final mass of 2.4 g. % change = ((2.4 − 3.2) ÷ 3.2) × 100 = (−0.8 ÷ 3.2) × 100 = −25% (mass decreased by 25%)
Why percentage change, not just change?
Different samples may have different starting values. Using percentage change allows fair comparison because it accounts for these differences. A mass increase of 0.5 g is a big change for a 1.0 g sample (50%) but a small change for a 10.0 g sample (5%). Percentage change makes this clear.
Frequently asked questions
Do I include the negative sign?
Yes: a negative percentage change means the value decreased. This is important information and should be included in your answer.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.