Circle Calculator

All tools

Enter any one of radius, diameter, circumference or area and instantly compute all the other circle properties. Free, accurate and easy to use.

r
Area –
Radius –
Diameter –
Circumference –
Area –
Calculations use π (pi). Diameter = 2r, circumference = 2πr and area = πr². Results are in the same unit as your input (area is squared).

About Circle Calculator

The circle calculator finds every property of a circle from a single measurement. Tell it which value you know — radius, diameter, circumference or area — and it instantly computes all the others, so you never have to rearrange a formula by hand.

Under the hood it uses the classic relationships: diameter = 2r, circumference = 2πr and area = πr². Whichever value you start from, the tool first solves for the radius and then derives the rest, keeping everything consistent to full precision.

It suits geometry homework, sizing round tables, pipes, pizzas or garden beds, and any job where you can measure one circle property but need another. The calculator is free and updates live as you type in your browser.

How to use Circle Calculator

  1. Choose which value you know: radius, diameter, circumference or area.
  2. Enter that single measurement.
  3. Read the other three properties, computed instantly.

Frequently asked questions

Yes. Select area as the known value and the calculator inverts A = πr² to get r = √(A/π), then reports the diameter and circumference from that radius.

Full double-precision π, not 3.14, so results are accurate to far more digits than any ruler or tape measure. Displayed values are rounded for readability.

The same unit as your input. If you enter a radius in centimeters, the diameter and circumference come out in centimeters and the area in square centimeters.

Circumference = π × diameter. Pick diameter as the known value, enter it, and the tool shows the circumference along with the radius and area.

Yes — completely free, with no signup. All the math runs locally in your browser, so nothing you type is sent anywhere.
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