Circle Calculator

Calculate a circle's area, circumference, diameter and radius from any one known value. Free and instant — results update as you type, entirely in your browser.

Any unit works: enter the radius in cm and you get the circumference in cm and the area in cm². π is used at full double precision (3.141592653589793…).

How to use the circle calculator

  1. Pick what you know. Choose radius, diameter, circumference or area from the dropdown — any single value is enough to define the whole circle.
  2. Type the value. Enter the number in any unit; decimals are fine. The other three properties are calculated instantly as you type.
  3. Copy the results. Read the radius, diameter, circumference and area, check the formulas with your numbers substituted in, and copy everything with one click.

About this tool

A circle is completely determined by a single measurement, so this circle calculator can start from whichever one you have: the radius, the diameter (d = 2r), the circumference (C = 2πr) or the area (A = πr²). Enter any of the four and it solves for the other three, showing the exact formula used with your numbers substituted in — perfect for checking geometry homework or learning how the equations rearrange (for example r = C / 2π, or r = √(A / π)).

Beyond the classroom, it answers practical questions in seconds: how much edging you need around a round flower bed, the area of a circular pizza or table top, the circumference of a wheel to work out distance per revolution, or the diameter of a pipe from its measured girth. All math runs locally in your browser with full double-precision π — nothing is uploaded, stored or logged, and it works offline once the page has loaded.

Frequently asked questions

What are the circle formulas used?

Diameter d = 2r, circumference C = 2πr (equivalently πd), and area A = πr². Working backwards: r = d / 2, r = C / (2π), and r = √(A / π). The calculator applies whichever rearrangement matches the value you entered.

How do I get the area from the circumference?

First find the radius: r = C / (2π). Then apply the area formula: A = πr². A shortcut that combines both is A = C² / (4π). Select “Circumference” in the tool and both steps are shown with your numbers.

What value of π does the calculator use?

It uses JavaScript's built-in double-precision value, 3.141592653589793 — accurate to about 16 significant digits. That is far more precise than the 3.14 or 22/7 approximations used in hand calculations.

What units should I enter?

Any unit you like. Lengths (radius, diameter, circumference) all share the unit you enter, and the area comes out in that unit squared — enter meters and the area is in m². Just don't mix units within one calculation.