The radius is half the diameter, so use the formula r = D/2. This is identical to the method used for calculating the radius of a circle from its diameter. If you have a sphere with a diameter of 16 cm, find the radius by dividing 16/2 to get 8 cm. If the diameter is 42, then the radius is 21. Use the formula C/2π. Since the circumference is equal to πD, which is equal to 2πr, dividing the circumference by 2π will give the radius.  If you have a sphere with a circumference of 20 m, find the radius by dividing 20/2π = 3.183 m. Use the same formula to convert between the radius and circumference of a circle. Use the formula ((V/π)(3/4))1/3. The volume of a sphere is  derived from the equation V = (4/3)πr3.  Solving for the r variable in this equation gets ((V/π)(3/4))1/3 = r, meaning that the radius of a sphere is equal to the volume divided by π, times 3/4, all taken to the 1/3 power (or the cube root.) If you have a sphere with a volume of 100 inches3, solve for the radius as follows:  ((V/π)(3/4))1/3 = r ((100/π)(3/4))1/3 = r ((31.83)(3/4))1/3 = r (23.87)1/3 = r  2.88 in = r Use the formula r = √(A/(4π)). The surface area of a sphere is derived from the equation A = 4πr2. Solving for the r variable yields √(A/(4π)) = r, meaning that the radius of a sphere is equal to the square root of the surface area divided by 4π. You can also take (A/(4π)) to the 1/2 power for the same result. If you have a sphere with a surface area of 1,200 cm2, solve for the radius as follows:  √(A/(4π)) = r √(1200/(4π)) = r √(300/(π)) = r √(95.49) = r  9.77 cm = r
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