Circle formulas
- Area: A = π × r²
- Circumference: C = 2 × π × r = π × d
- Diameter: d = 2 × r
π (pi) ≈ 3.14159 is the ratio of any circle's circumference to its diameter.
Worked examples
A pizza 30 cm across has r = 15 cm and an area of π × 15² ≈ 706.9 cm². Two 20 cm pizzas have only 2 × π × 10² ≈ 628.3 cm², less than one 30 cm pizza.
A round table 12 in across covers π × 6² ≈ 113.1 in². A circular garden bed with a 31.4 m edge has a radius of 31.4 ÷ 2π ≈ 5 m and an area of about 78.5 m².
Working backwards
From an area you can recover the radius: r = √(A ÷ π). A 100 m² circular plot has a radius of about 5.64 m.
How this works
The page converts whichever measurement you give into the radius (half the diameter, the circumference divided by 2π, or the square root of the area divided by π) and then computes the area as πr², the circumference as 2πr and the diameter as 2r. π is used to full double precision. Area results are labelled with the square of your unit. Everything runs in your browser.
Frequently asked
What is the formula for the area of a circle?
A = πr², where r is the radius (half the diameter).
How do I find the radius from the circumference?
Divide by 2π: a 62.8 cm circumference gives a radius of about 10 cm.
What is the difference between area and circumference?
Area is the surface inside the circle (in square units); circumference is the length around it.
How accurate is π here?
To about 15 decimal places, far beyond any physical measurement.
Sources
Reviewed Oct 17, 2026 · test vectors fixtures/calculators/math/area/circle/