WorldCalcs

Circumference Calculator

Find a circle's circumference from its radius, diameter, area, or circumference.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: July 2026

Circumference

31.4159

C = 2πr

Radius

5

r

Diameter

10

d = 2r

Area

78.5398

A = πr²

What is circumference?

The circumference is the distance all the way around a circle — its perimeter. This calculator finds the circumference from whatever you already know: the radius, the diameter, the area, or even the circumference itself (to recover the other measurements). Enter one value and it fills in the rest.

How to find circumference

Every measurement of a circle flows from its radius, so the formulas connect back to it:

  • From the radius: Circumference = 2 × π × r, Diameter = 2 × r, Area = π × r²
  • From the diameter: Circumference = π × d (because the diameter is twice the radius)
  • From the circumference: radius = C ÷ (2π), diameter = C ÷ π
  • From the area: radius = √(A ÷ π), then use the radius formulas

Here π (pi) ≈ 3.14159265 — the fixed ratio of every circle's circumference to its diameter.

Example

A circle with a radius of 5 has a circumference of 2 × π × 5 = 31.4159, a diameter of 10, and an area of π × 5² = 78.5398. A circle with a diameter of 7 has a circumference of π × 7 = 21.9911. Because each value traces back to the radius, knowing any one of them is enough to find the others.

Related: get all four measurements from the radius in the Circle Calculator, take square roots for the area-to-radius step in the Square Root Calculator, measure floor space with the Square Footage Calculator, or browse more shapes on Geometry Calculators.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Choose which measurement you know and enter its value. The calculator returns the circumference and fills in the other three: radius, diameter and area.

Every value ties back to the radius through π ≈ 3.14159, so any one of them is enough to find the rest.

Frequently asked questions

What is the formula for circumference?+

C = 2πr, where r is the radius, or the equivalent C = πd, where d is the diameter.

How do I find circumference from the diameter?+

Multiply the diameter by π. A diameter of 7 gives a circumference of 7 × 3.14159 = 21.9911.

How do I find the radius from the circumference?+

Divide the circumference by 2π. A circumference of 31.4159 gives 31.4159 ÷ 6.28319 = 5.

What is the difference between circumference and perimeter?+

Circumference is the specific name for the perimeter of a circle. For straight-sided shapes we simply say "perimeter."

What value of pi should I use?+

π ≈ 3.14159265. This calculator uses the full-precision value built into your device, so the results are as accurate as possible.

How is circumference related to area?+

Both come from the radius: C = 2πr and A = πr². Knowing one lets you find the radius and then the other, which this calculator does for you automatically.

What units does circumference use?+

The same length unit you enter for the radius or diameter — centimetres in, centimetres out. Area is reported in those units squared.

Why is the circumference divided by the diameter always pi?+

Because that ratio is the definition of π. It is the same for every circle, no matter how big or small.

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