How to Find the Radius of a Circle If You Have the Area

Find the radius of a circle when you know the area.

When you measure the area of a shape, you measure the size of what's enclosed by the perimeter. The radius of a circle is the distance from its center to any point on the outside edge. If you know the area of a circle, you have enough information to figure the radius by reversing the formula for figuring area.

Things You'll Need

• Paper
• Pencil
• Calculator

Instructions

1. Understand that the formula for figuring the area of a circle is 3.14 (pi) multiplied by the radius squared. For example, if the radius is 8, to figure the area, you multiply 3.14 times 8 times 8 to equal 200.96.

2. Begin with the area measurement to figure the radius of a circle. Divide the area measurement by 3.14 (pi). Using the same example, if the area is 200.96, divide this number by 3.14 to equal 64.

3. Finish reversing the formula by figuring the square root of the answer from the previous step. Using the same example, figure the square root of 64 is 8. This means that 8 is the radius of a circle with an area of 200.96.

References

• Cerritos College: Circumference of a Circle