SuperCalX

Circle Calculator

Find the radius, diameter, circumference and area of a circle from any one of them.

Calculator by SuperCalX: formulas, worked examples and 250+ free calculators.

Your values

Result

Radius
5
Diameter
10
Circumference
31.415927
Area
78.539816 units²

Result breakdown

The figures from your result, side by side
radius
5
diameter
10
circumference
31.416
area
78.54

The largest figure is area at 78.54. It is about 16 times larger than radius (5).

Change any input and these figures update straight away. Open the Charts tab to see them as a graph; the formula and a worked example are further down this page.

How to Use

Step-by-step instructions
  1. 1Choose which measurement you know.
  2. 2Enter its value in any unit.
  3. 3Read the other three measurements in the same unit (area in square units).

Circle Formulas

Every measurement of a circle follows from its radius, so the calculator converts your known value to a radius first.
d = 2r, C = 2πr, A = πr²

Variables:

rRadius
dDiameter
CCircumference
AArea

Example

Radius of 5 cm

Inputs:

Radius:5 cm

Steps:

  1. 1.Diameter = 2 × 5 = 10 cm
  2. 2.Circumference = 2 × π × 5 = 31.416 cm
  3. 3.Area = π × 5² = 78.540 cm²
Result:
d = 10 cm, C = 31.42 cm, A = 78.54 cm²

Circle Calculator: the short answer

Every circle measurement follows from the radius r: diameter d = 2r, circumference C = 2πr and area A = πr². A circle with a 5 cm radius has a 10 cm diameter, a 31.42 cm circumference and a 78.54 cm² area. To work backwards, r = C ÷ 2π or r = √(A ÷ π).

  • π ≈ 3.14159; it is the ratio of circumference to diameter for every circle.
  • Doubling the radius doubles the circumference but quadruples the area.
  • Area units are squared: cm², m², sq ft.

By the SuperCalX team · Reviewed by Dr. Ahmed Mortuza Saleque, founder and editor · Last reviewed · How we check our calculators · También en español

Circle formulas

FindFrom radiusFrom diameterFrom circumferenceFrom area
Radiusrd ÷ 2C ÷ 2π√(A ÷ π)
Diameter2rdC ÷ π2√(A ÷ π)
Circumference2πrπdC2√(πA)
Areaπr²πd² ÷ 4C² ÷ 4πA

Where circle maths comes up

Worked examples

Radius 5 cm

  1. d = 2 × 5 = 10 cm.
  2. C = 2 × π × 5 = 31.416 cm.
  3. A = π × 25 = 78.540 cm².

d = 10 cm, C = 31.42 cm, A = 78.54 cm².

Circumference 100 cm

  1. r = 100 ÷ 2π = 15.915 cm.
  2. A = π × 15.915² = 795.8 cm².

Radius 15.92 cm, area 795.8 cm².

Common mistakes

  • Using the diameter in A = πr²; halve it first.
  • Forgetting to square the units for area.

Circle Calculator FAQ

How do I find the area of a circle?

Square the radius and multiply by π. A 3 m radius gives π × 9 = 28.27 m².

How do I find the radius from the circumference?

Divide the circumference by 2π. A 62.83 cm circumference has a radius of 10 cm.

What is the area of a semicircle?

Half the full area: πr² ÷ 2.

What is π?

Pi is the ratio of a circle's circumference to its diameter, about 3.14159. It is the same for every circle.

Sources

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