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
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.
Why pi is irrational
Pi (about 3.14159…) never terminates or repeats, so it cannot be written exactly as a fraction — it is what mathematicians call an irrational number. Any measurement that uses pi, like a circle's circumference or area, is therefore always an approximation at some level of precision, however many decimal places you use.