Circle Calculator
Radius, diameter, circumference and area from any one of them
Your Circle
Formulas
- d = 2r
- C = 2πr = πd
- A = πr²
- r = 7 cm → C = 43.982297 cm, A = 153.93804 cm²
- 1Pick what you knowRadius, diameter, circumference or area
- 2Enter the valueIn cm, m, inches or feet
- 3Get the restAll four, with the formulas
Circle formulas
Every measurement of a circle follows from its radius r and the constant π ≈ 3.14159:
Diameter d = 2r
Circumference C = 2πr
Area A = πr²
r = C ÷ 2π = √(A ÷ π)
Using 22/7 for π
School textbooks often use π = 22/7 ≈ 3.1429, which makes r = 7 cm give a neat area of 154 cm². This calculator uses the exact value of π, so answers can differ slightly in the last decimal places.
Finding the radius from the circumference
Wrap a string or tape around a round object to measure its circumference, then choose “Circumference”. Dividing by 2π gives the radius, which is easier than measuring through the centre.
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Frequently asked questions
How do I find the area of a circle?
Square the radius and multiply by π. With r = 7 cm, A = π × 49 ≈ 153.94 cm².
How do I find the circumference from the diameter?
Multiply the diameter by π. A 10 cm circle has a circumference of about 31.42 cm.
What is the difference between radius and diameter?
The radius goes from the centre to the edge. The diameter goes right across through the centre, so it is twice the radius.
Last reviewed: October 2026