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Circle Calculator

Solve a circle from radius, diameter, circumference or area.

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Enter any one value

The other three are worked out from it.

Arcs, sectors and chords

Radius—
Diameter—
Circumference—
Area—

Working backwards

Getting area from a radius is easy. Going the other way means rearranging to r = √(A ÷ π), which is the part worth having a calculator for.

Quick Tool Overview

The Circle Calculator takes any single property of a circle — radius, diameter, circumference or area — and works out the other three, each with the formula it used. A second panel covers arcs, sectors, chords and segments.

What it does
Solves a circle from any one known value, plus arcs and sectors
You give it
A radius, diameter, circumference or area
You get back
The other three values, with arc, sector and chord figures
Best for
Geometry homework, engineering, DIY and pipe or tank work
Calculators Free, unlimited use No sign-up Runs in your browser

About the Circle Calculator

Enter any single property of a circle — the radius, diameter, circumference or area — and the other three are worked out from it, each with the formula it used.

Working backwards is where this earns its place. Getting the area from a radius is easy to do in your head; getting the radius from a known area means rearranging to the square root of the area over pi, which is less pleasant. A second panel covers the parts of a circle: arc length and sector area from a central angle in degrees or radians, plus chord length and the area of the segment cut off by it.

Key features of the Circle Calculator

Any input, all outputs

Enter whichever value you happen to know and the rest follow.

Works backwards cleanly

Getting the radius from a known area means the square root of area over pi — unpleasant by hand, instant here.

Arc length and sector area

From a central angle in degrees or radians, without converting first.

Chords and segments

Chord length and the area of the segment it cuts off are included, which most circle calculators omit.

Formula shown for each result

Every value prints the rearrangement used, so the page doubles as a reference.

How to use the Circle Calculator

  1. 1

    Enter any one value

    Type the radius, diameter, circumference or area — whichever you already know.

  2. 2

    Read the other three

    They are calculated instantly, each with the formula used shown beside it.

  3. 3

    Work out arcs and sectors

    Add a central angle to get the arc length, sector area, chord length and segment area.

Frequently asked questions

Pi times the radius squared. A circle of radius 5 has an area of pi x 25, which is about 78.54 square units.

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