About this calculator
Enter one measurement of a circle and this calculator finds the other three: the radius, diameter, circumference and area.
Use any unit you like (centimetres, inches, metres). The answers are in the same unit, and the area in that unit squared.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
Radius of 5
- Diameter
- 10
- Radius
- 5
- Circumference
- 31.415927
- Area
- 78.539816
A circle with a radius of 5 has a radius of 5, a diameter of 10, a circumference of 31.415927 and an area of 78.539816.
Show the working
- Diameter = 2 × radius = 2 × 5 = 10.
- Circumference = 2πr = 2 × 3.141593 × 5 = 31.415927.
- Area = πr² = 3.141593 × 5² = 78.539816.
Diameter of 10
- Radius
- 5
- Diameter
- 10
- Circumference
- 31.415927
- Area
- 78.539816
A circle with a diameter of 10 has a radius of 5, a diameter of 10, a circumference of 31.415927 and an area of 78.539816.
Show the working
- Radius = diameter ÷ 2 = 10 ÷ 2 = 5.
- Circumference = 2πr = 2 × 3.141593 × 5 = 31.415927.
- Area = πr² = 3.141593 × 5² = 78.539816.
Area of 100
- Radius
- 5.641896
- Diameter
- 11.283792
- Circumference
- 35.449077
- Area
- 100
A circle with an area of 100 has a radius of 5.641896, a diameter of 11.283792, a circumference of 35.449077 and an area of 100.
Show the working
- Radius = √(area ÷ π) = √(100 ÷ 3.141593) = 5.641896.
- Circumference = 2πr = 2 × 3.141593 × 5.641896 = 35.449077.
- Area = πr² = 3.141593 × 5.641896² = 100.
The circle formulas
- Diameter: d = 2r
- Circumference: C = 2πr = πd
- Area: A = πr²
π (pi) is about 3.14159265, the ratio of any circle's circumference to its diameter.
Working backwards
If you know the circumference, the radius is C ÷ 2π. If you know the area, the radius is √(A ÷ π). Once you have the radius, everything else follows from the formulas above.
Circle facts worth knowing
Doubling a circle's radius doubles its circumference but quadruples its area, because area depends on the radius squared. A circle of diameter 10 has an area of about 78.54 and a circumference of about 31.42.
Frequently asked questions
What is the formula for the area of a circle?
Area = π × radius squared (A = πr²).
How do I find the radius from the circumference?
Divide the circumference by 2π (about 6.2832).
What is the difference between radius and diameter?
The radius runs from the centre to the edge. The diameter runs all the way across through the centre, so it is twice the radius.
What value of π does it use?
The full precision of your device (3.141592653589793), so the answers are accurate to many more digits than shown.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.