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Circle Calculator

Enter the radius, diameter, circumference or area and get all the others, plus the arc length and sector area for any angle you choose.

Formulas last reviewed: September 2026

Disclaimer: Results are calculated with standard formulas and are meant for learning and quick checks. Please double-check important work, especially anything used for exams, engineering, finance or safety.

Everything about a circle from one measurement

Every measurement of a circle is linked to the radius, so knowing any one of them, the radius, diameter, circumference or area, gives you the rest. This calculator does the conversion in one step.

It also finds the arc length and the area of a sector for any angle you enter, which is useful for pizza slices, pie charts, curved paths and machine parts.

How it is calculated

Diameter = 2 × radius
Circumference = 2 π r
Area = π r²
Arc length = angle ÷ 360 × circumference

Worked example

Radius 7

Diameter
14
Circumference
43.98
Area
153.94
A 90° sector has area
38.48

About π

Pi is the ratio of any circle's circumference to its diameter, about 3.14159. For everyday work 3.14 or 22/7 is close enough, but this calculator uses a far more accurate value.

  • Doubling the radius doubles the circumference but quadruples the area.
  • A full turn is 360 degrees or 2π radians.

Frequently asked questions

What is the difference between radius and diameter?

The radius runs from the centre to the edge, and the diameter runs all the way across through the centre, so it is twice the radius.

How do I find the radius from the circumference?

Divide the circumference by 2π. The calculator does this when you pick circumference as the known value.

What is a sector?

A sector is a slice of a circle between two radii, like a slice of pizza. Its area is a fraction of the whole circle.

Why does the area grow faster than the circumference?

Area depends on the radius squared, while circumference depends on the radius itself, so area grows much faster.