The diameter of a circle is the distance across a circle directly through its center. The radius is one half of the diameter in measurement. The radius measures the distance from the very center of the circle to any point on the circle. You can calculate either of the measurements if you have the circumference of a circle. Circumference is the total distance around a circle. The circumference of a circle equals the diameter of the circle multiplied by pi, which is 3.14159.
Take the circumference of a circle and divide it by Pi. For example, if the circumference is 12.56, you would divide 12.56 by 3.14159 to get 4, which is the diameter of the circle.
Use the diameter to find the radius by dividing the diameter by 2. For example, if the diameter is 4, the radius would be 2.
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Check your calculations for accuracy. Work through your results backward to verify your findings. Use “D = R x 2”, in which "D" equals diameter and "R" equals radius, to solve for the diameter using the numbers you got earlier. Then use “C = pi x D”, in which "C" equals circumference, to solve for the circumference. If everything checks out, your calculations are correct, but if not, you need to work through the formulas again from the start to look for your error.