Circle calculator
A circle's area is π times the radius squared; its circumference is two π times the radius. Every measure of a circle flows from one number — the radius — so knowing any one value yields all the rest.
- Circumference
- 31.416
- Diameter
- 10
Rates and thresholds
- Area
- S = πr²
- Circumference
- C = 2πr = πd
- Diameter
- d = 2r
- π
- ≈ 3.14159
How do I calculate the area of a circle?
Multiply π by the radius squared: S = πr². With a 5 cm radius the area is 3.14159 × 25 ≈ 78.5 cm². If you know the diameter, halve it first — a 10 cm diameter circle has a 5 cm radius. The classic mistake is squaring the diameter, which gives an answer four times too large.
How do I find the circumference from the area?
Find the radius first: r = √(S/π). Then the circumference C = 2πr. A circle of 100 m² has radius √(100/3.14159) ≈ 5.64 m and circumference 2 × π × 5.64 ≈ 35.4 m. The calculator does both steps at once, whichever value you start from.
How many square metres is a round area?
Measure the diameter, halve it, and apply πr². A three-metre pool covers π × 1.5² ≈ 7.1 m²; a six-metre trampoline, π × 3² ≈ 28.3 m². When buying material — sand, soil, pavers — add roughly a tenth for measurement error and waste.
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