Calculating cross-sectional areasSpec 5.19B
In short
The area of a clear zone or bacterial colony is calculated with the equation for the area of a circle, area = πr², because they are roughly circular. The radius, r, is half the diameter, so measure the diameter, halve it, square it and multiply by π. A clear zone 14 mm across has an area of about 154 mm².
Clear zones and bacterial colonies are roughly circular, so their area is calculated using the equation for the area of a circle. A larger area means a bigger effect (for a clear zone) or more growth (for a culture).
area = πr²
The radius, r, is half the diameter. Measure the diameter across the middle of the zone, halve it, then square it and multiply by π.
Area of a clear zone
The clear zone around a disc soaked in antiseptic has a diameter of 14 mm. Calculate the area of the clear zone. Use π = 3.14.
- radius = 14 ÷ 2 = 7 mm
- area = πr² = 3.14 × 7 × 7
- area = 3.14 × 49 = 153.86 mm²
Answer: About 154 mm²
Maths skill:A very common slip is to use the diameter instead of the radius. Halve the diameter first, and give your answer in mm² (or cm²).
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026
Frequently asked questions
Why can't antibiotics kill viruses?
Antibiotics cannot kill viruses because they work by inhibiting cell processes in bacteria, and viruses are not cells. Viruses live and reproduce inside host cells, so they do not have the cell processes that antibiotics target. This is why antibiotics cannot be used to treat viral diseases such as HIV, colds or flu.
What are the stages of developing a new medicine?
New medicines go through four stages: discovery of a chemical that might treat the disease, development to improve it, preclinical testing on cells, tissues and live animals to check toxicity and effectiveness, and clinical testing on people. Clinical testing starts with a few healthy volunteers to check safety, then patients to find the dose.
Why are bacterial cultures incubated at 25 °C?
Triple only Bacterial cultures are incubated at 25 °C because a lower temperature makes it less likely that harmful pathogens, which grow best at body temperature, will grow. The lid is taped on but not fully sealed, so microorganisms cannot escape or get in, but oxygen can still enter. The dish is incubated upside down.
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