Formula
circumference C = 2πrarea A = πr²diameter d = 2r
- r — radius
- π ≈ 3.14159265358979
Everything about a circle follows from its radius.
Worked examples
Radius 7
- I know the circle's…
- Radius
- Value
- 7
- Unit
- No unit
Area
153.93804003
Radius: 7
Diameter: 14
Circumference: 43.98229715
Circumference 100
- I know the circle's…
- Circumference
- Value
- 100
- Unit
- cm
Area
795.77471546 cm²
Radius: 15.91549431 cm
Diameter: 31.83098862 cm
Circumference: 100 cm
How it works
A circle is determined by its radius. The diameter is twice the radius, the circumference (distance around) is 2πr and the area is πr². If you know a different measurement, the calculator first works back to the radius — dividing the diameter by 2, the circumference by 2π, or taking the square root of the area divided by π — and then finds the rest.
A circle with radius 7 has a diameter of 14, a circumference of about 43.98 and an area of about 153.94. Units matter: if the radius is in centimetres, the circumference is in centimetres and the area in square centimetres, and the optional unit selector labels the area accordingly.
For a circle inside other figures use the Area and Volume Calculator, which also handles cylinders, cones and spheres. Triangles are covered by the Triangle Calculator, and the Square Root Calculator helps if you need the radius from an area by hand.
Assumptions and limits
- Uses π to double precision (about 15 digits).
- All values use the same unit.