Circle Calculator
A circle has only one degree of freedom, so a single measurement fixes all the others. Give this calculator a radius, a diameter, a circumference or an area and it recovers the radius first, then rebuilds every remaining figure from it.
- Accurate
- Real-time
- Easy to use
- 100% free
Area
314.159265
Diameter
20.000000
Details
Updates as you typeThe one circle measurement you already have. Must be greater than zero.
Summary
Area
314.159265
- Radius
- 10.000000
- Diameter
- 20.000000
- Circumference
- 62.831853
- All four outputs share whatever length unit you typed in: a radius in centimetres gives an area in square centimetres.
- Area grows with the square of the radius, so doubling the radius quadruples the area while only doubling the circumference.
How this is calculated
- Known radius
- 10.000000
- Radius r
- 10.000000
- Diameter 2r
- 20.000000
- Circumference 2πr
- 62.831853
- Area πr²
- 314.159265
Compare scenarios
See how one change moves the result
- CurrentYour inputs as they stand314.159265Current
- Known measurement12.5490.873852
Frequently asked questions
How do I find the radius from the area of a circle?
Rearrange A = πr² to r = √(A ÷ π). For an area of 50 square metres that is √(50 ÷ 3.14159…) ≈ 3.9894 metres. This calculator does exactly that whenever you select "Area" as the known measurement.
How do I find the radius from the circumference?
Divide the circumference by 2π. A circumference of 62.83 units gives 62.83 ÷ 6.28318… ≈ 10 units of radius. Once the radius is known, the diameter is 2r and the area is πr².
What units does the answer come out in?
The same units you typed in. Radius, diameter and circumference are lengths, so they share your input unit; the area is that unit squared. Enter centimetres and the area is in square centimetres.
Why does doubling the radius quadruple the area?
Area depends on the radius squared, so multiplying the radius by two multiplies the area by two squared, which is four. Circumference depends on the radius only to the first power, so it merely doubles.