# Radius of a Circle

LAST UPDATE: February 2nd, 2022

## Method #1 – Radius from Area

### Formula – How to calculate the radius of a circle using the area

π approximately equal to 3.1415926535

### … from the Area

Radius = √(Area ÷ π)

##### Example

A circle has an area of 10

Radius = √(10 ÷ π) = √(10 ÷ 3.14159) = √(3.18310) = 1.7841

### Formula – How to calculate the radius of a circle using the circumference

π approximately equal to 3.1415926535

A circle has a circumference of 10

##### Example

Radius = 10 ÷ π ÷ 2 = 10 ÷ 3.14159 ÷ 2 = 3.18301 ÷ 2 = 1.5915

## Method #3 – Radius from Diameter

### Formula – How to calculate the radius of a circle using the diameter

π approximately equal to 3.1415926535

Radius = Diameter ÷ 2

##### Example

A circle has a diameter of 10

Radius = 10 ÷ 2 = 5