Circle Calculator
Enter any one circle measurement — radius, diameter, circumference or area — and get the other three instantly, with all formulas shown.
About the Circle Calculator
This circle calculator solves the whole circle from any single measurement. Tell it which value you know — radius, diameter, circumference or area — enter the number, and it instantly computes the other three. Every result uses the exact value of π, and the formulas used are displayed below the answers so you can follow the working.
The relationships are simple once you know the radius: diameter d = 2r, circumference C = 2πr, and area A = πr². Working backwards, r = C ÷ 2π from a circumference, and r = √(A ÷ π) from an area. Students meet these formulas from middle school onwards, and they show up constantly in real life — sizing a round table cover, a pizza, a water tank, a garden bed or a pipe cross-section.
The calculator is unit-agnostic: enter centimetres and the answers are in centimetres (area in cm²), enter metres and everything is in metres. It runs entirely in your browser with no signup.
How to Use the Circle Calculator
- 1Choose which measurement you know: radius, diameter, circumference or area.
- 2Type the value (any unit — results use the same unit).
- 3Read the other three measurements instantly.
- 4Check the formulas panel to see how each value was derived.
Frequently Asked Questions
How do I find the area of a circle from the radius?
Use A = πr². For a radius of 5 cm, the area is π × 5² = 78.54 cm². If you have the diameter instead, halve it first: a 10 cm diameter circle has radius 5 and the same 78.54 cm² area.
How do I find the radius from the circumference?
Divide the circumference by 2π. A circle with a 31.42 cm circumference has radius 31.42 ÷ 6.2832 = 5 cm. This is handy when you can wrap a tape measure around something round but cannot measure across it — like a pipe or a tree trunk.
How do I calculate the diameter from the area?
First find the radius with r = √(A ÷ π), then double it. For an area of 100 cm², r = √(100 ÷ 3.1416) = 5.64 cm, so the diameter is 11.28 cm. The calculator does both steps for you when you select 'Area' as the known value.
What value of pi does this circle calculator use?
It uses JavaScript's built-in double-precision value of π (3.141592653589793), which is far more accurate than the 3.14 or 22/7 approximations used in hand calculations. Results are displayed to 4 decimal places.
What is the circumference of a circle with a diameter of 12?
Circumference = πd = 3.1416 × 12 = 37.70 (same unit as the diameter). Select 'Diameter' in the calculator, enter 12, and you also get the radius (6) and area (113.10 square units) at the same time.