Calculating the Diameter of a Circle Using the Radius, Circumference, or Area
If you know the radius of the circle, double it to get the diameter. The radius is the distance from the center of the circle to its edge. ExampleIf the radius of the circle is 4 cm, then the diameter of the circle is 4 cm x 2, or 8 cm.
If you know the circumference of the circle, divide it by π to get the diameter. π is equal to approximately 3.14 but you should use your calculator to get the most accurate results. ExampleIf the circumference of the circle is 10 cm, then the diameter is 10 cm/π, or 3.18 cm.
If you know the area of the circle, divide the result by π and find its square root to get the radius; then multiply by 2 to get the diameter. This goes back to manipulating the formula for finding the area of a circle, A = πr, to get the diameter. You can transform this into r = √(A/π) cm. ExampleIf the area of the circle is 25 cm, divide that by π and find the square root.This equals cm 2.82cm, so the diameter of the circle is 2.82 x 2 = 5.64cm.
Calculating the Diameter of a Circle from a Drawing of a Circle
Draw a horizontal line inside the circle from one edge to the other. Use a ruler or a straight edge to do this. It can be at the top, near the bottom, or anywhere in between.
Label the points where the line crosses the circle points "A" and "B."
Draw two overlapping circles, one using A as the center and the other using B as the center. Be sure the two circles overlap like a Venn diagram.
Draw a vertical line through the two points at which the outer circles intersect. This line marks the diameter of the circle.
Measure the diameter. Measure it using a ruler, or for greater accuracy, a pair of digital calipers. You're done!
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