CalcArcade.

Circle Calculator

Pick the measurement you know, enter it, and the calculator works out the other three.

Your numbers

Result

How we calculate it

Everything is found from the radius r. The diameter is 2 × r, the circumference is 2 × π × r and the area is π × r². If you know another measurement first, the radius is diameter ÷ 2, circumference ÷ (2 × π) or √(area ÷ π).

For a radius of 5, the diameter is 10, the circumference is 31.42 and the area is 78.54.

Assumptions and limits

  • Use any unit, as long as you read the answers in that unit. Area is in that unit squared.
  • π is used at the full accuracy of a standard number, not as 3.14.
  • The value must be above zero.

Frequently asked questions

How do I find the area of a circle from its diameter?

Halve the diameter to get the radius, then multiply π by the radius squared. A diameter of 10 gives a radius of 5 and an area of about 78.54.

What is the circumference of a circle?

It is the distance around the circle. It equals π times the diameter, or 2 times π times the radius.

How do I find the radius from the area?

Divide the area by π and take the square root. An area of 100 gives a radius of about 5.642.

Related calculators

Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: I know the = Radius; Value = 5.

  • Area 78.5398
  • Radius 5
  • Diameter 10
  • Circumference 31.4159

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.