Circle Calculator
Enter any one of radius, diameter, circumference or area to get all the others.
About the Circle Calculator
Enter any one measurement of a circle (radius, diameter, circumference or area). The calculator gives all the others with the working.
Every circle is fixed by one measurement. π (pi ≈ 3.14159265) is the ratio of a circle's circumference to its diameter, the same for every circle.
The formula
C = 2πr = πd; A = πr²
- r
- radius
- d
- diameter = 2r
- π
- 3.14159265358979…
Worked example
Circumference 31.4 cm
- r = 31.4 ÷ 2π ≈ 5.0 cm
- A ≈ 78.5 cm²
Questions people ask
Is 22/7 accurate for π?
It's 3.142857, about 0.04% too big: fine for rough work. This calculator uses π to 15 digits.
How do I measure a circle's diameter?
Measure across the widest point through the centre, or measure the circumference with a tape and divide by π.
How do I find the radius from the circumference?
Divide by 2π: a 31.4 cm circumference means a 5 cm radius.