Circle Calculator guide
Circle Calculator: Radius, Diameter, Circumference and Area
Use our circle calculator to find a circle's radius, diameter, circumference and area from any one of them.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
A circle has four key measurements, and if you know one, you can find the rest. Our Circle Calculator does it instantly.
How to use the calculator (Circle Calculator)
- Choose what you know: radius, diameter, circumference or area.
- Enter the value and choose your units.
The formulas
- Diameter d = 2r
- Circumference C = 2πr = πd
- Area A = πr²
π (pi) is about 3.14159. It's the ratio of any circle's circumference to its diameter.
To work backwards:
- r = C ÷ 2π
- r = √(A ÷ π)
A worked example
With the calculator's default settings, a circle with a radius of 5cm has:
- Diameter: 10cm
- Circumference: 31.42cm
- Area: 78.54cm²
Frequently asked questions
What's the difference between radius and diameter?
The radius goes from the centre to the edge. The diameter goes all the way across through the centre, so it's twice the radius.
How do I measure the circumference of something round?
Wrap a piece of string around it, then measure the string. Enter that as the circumference to find the diameter.
What about part of a circle?
For a sector (a pizza slice), use our Area Calculator.
Sources and further reading
- GOV.UK: National curriculum in England – mathematicsgov.uk
- Wolfram MathWorld: Circlemathworld.wolfram.com
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.