How to Find the Length of the Radius of a Circle

The radius of a circle is the distance from the center to any point located on the circle. Computing various geometric parameters of a circle including the radius is a common type of math problem. To find out the radius length you need to know at least one of the following values: the circle diameter, the circle circumference or the area of a circle.

Things You'll Need

• Calculator

Instructions

1. Get the constant "pi" with the required precision (see references) that is necessary for calculations of the circle geometry; the pi constant rounded to thousands, 3.142 is sufficient for most calculations.

2. Divide the circle diameter by two to find out the radius. For example, if the diameter is 11 inches then the radius is 11 / 2 = 5.5 inches.

3. Divide the circle circumference by the constant pi, and then by two to find out the radius of the circle. For example, if the circumference is 17.5 inches, then the radius is 17.5 / (3.142 * 2) = 2.785 inches.

4. Divide the circle area by the constant pi, and then take the square root to calculate the radius. For example, if the circle area is 27 square inches, then the radius is square root (27 / 3.142) = square root ( 8.593) = 2.931 inches.

References

• Cool Math: The Geometry of Circles
• Wolfram Math World: Pi