Method #1 – Radius from Area
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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
Method #2 – Radius from Circumference
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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
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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