Calculatoo

Circle Calculator: Area, Circumference, Radius, Diameter

Choose which measurement you know, enter it, and the calculator works out the radius, diameter, circumference and area.

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Formula

  • diameter = 2 × radius
  • circumference = 2 × π × radius
  • area = π × radius²

Starting from another measurement: radius = circumference ÷ (2π) or radius = √(area ÷ π).

Worked example

Radius 5:

  • Diameter = 2 × 5 = 10
  • Circumference = 2 × π × 5 = 31.4159
  • Area = π × 5² = 78.5398

From an area of 100: radius = √(100 ÷ π) = 5.6419.

Frequently asked questions

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

Multiply π by the radius squared. For a radius of 5, the area is π × 25 = 78.5398 square units.

How do I find the circumference from the diameter?

Multiply the diameter by π. A circle with diameter 10 has circumference 10 × π = 31.4159.

How do I get the radius from the area?

Divide the area by π and take the square root. For an area of 100, r = √(100 ÷ π) = 5.6419.

What units does the answer use?

Any, as long as you stay consistent. Lengths come out in the unit you entered and the area in that unit squared, so a radius in centimetres gives an area in square centimetres.

How we calculate

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