How to Determine the Diameter of a Circle

By Rachel Pancare; Updated April 24, 2017
Use a ruler, protractor or calculator to help you find the diameter.

You can find the diameter of a circle by using other measurements of the circle. Before you begin calculating, be certain you understand the parts of a circle. The diameter is the distance across the circle from a point on one side to a point on the opposite side, passing through the exact middle point of the circle. You can use the radius, circumference or area to help you find this measurement.

Understanding the Parts of a Circle

Reviewing the parts of a circle gives you the foundation needed to make diameter calculations. and be certain you know the meaning of diameter. Remember, the diameter is a straight line segment that passes through the center of the circle. It is the longest chord of the circle -- a line segment that joins two points on a curve.

The radius measurement is the easiest measurement to use to find the diameter of a circle. The radius is half of the diameter. The circumference of a circle is the distance around the outer edge of the circle from one point, meeting back at that point. The area of a circle is the space inside the circle.

Using the Radius

A simple calculation using the radius can help you determine the diameter of a circle. The radius is the distance from the center of the circle to its edge. The radius is equal to half of the diameter: radius + radius, or radius x 2, = diameter. If the radius of a circle is 8 centimeters, the diameter of that circle is 16 centimeters. If the radius of a circle is 20 inches, the diameter of the circle is 40 inches.

Using the Circumference

The circumference of a circle, the distance around the circle, requires a slightly more complex calculation. You can divide this measurement by π (roughly 3.14) to get the diameter. Use this formula: circumference = 3.14 x diameter. If the circumference of a circle is 5, then divide 5 by 3.14. The diameter of this circle is 1.59.

Using the Area

You can use the area of a circle to find its diameter by first finding the radius. If you know the area of the circle, find its square root. Then divide by 3.14 to get the radius. Once you have the radius, you can multiply by 2 to get the diameter.

For instance, if the area of the circle is 25 square centimeters, find the square root: √25 = 5 cm. Next divide by 3.14 using the equation 5 cm/π =1.59 cm. The radius of the circle is 1.59 cm. Multiply the radius by 2. The diameter of the circle is 1.59 x 2, which equals 3.18.

About the Author

Rachel Pancare taught elementary school for seven years before moving into the K-12 publishing industry. Pancare holds a Master of Science in childhood education from Bank Street College and a Bachelor of Arts in English from Skidmore College.