How it’s calculated
Everything about a circle follows from its radius r (centre to edge). The diameter is twice the radius, and π (≈ 3.14159) is the ratio of circumference to diameter.
Example: a circle with radius 5 in has diameter 10 in, circumference 2 × π × 5 ≈ 31.42 in and area π × 5² ≈ 78.54 in².
Frequently asked questions
What is the formula for the area of a circle?
A = πr², where r is the radius. If you know the diameter, use A = π(d/2)².
How do I find the diameter from the circumference?
Divide the circumference by π. A 31.42 in circumference gives a diameter of about 10 in.
Is a 16-inch pizza twice as big as an 8-inch pizza?
It is four times as big. Area grows with the square of the diameter: (16/8)² = 4.
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Sources
- Prealgebra 2e, 9.5 Solve Geometry Applications: Circles and Irregular Figures — OpenStax
- Circle — Wolfram MathWorld
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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