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Circle Calculator
Solve a circle from radius, diameter, circumference or area.
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
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
Enter any one value
Type the radius, diameter, circumference or area — whichever you already know.
-
2
Read the other three
They are calculated instantly, each with the formula used shown beside it.
-
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.
Divide the circumference by 2 pi. A circumference of 31.4 gives a radius of about 5.
Divide the area by pi, then take the square root. An area of 78.54 gives 25, and the square root of that is a radius of 5.
Multiply the circumference by the central angle divided by 360, if the angle is in degrees. In radians it is simply the radius times the angle.
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