The radius (r) is the distance from the exact center of the sphere to any point on the surface of the sphere. Generally speaking, you can find the radius of a sphere if you know the diameter, the circumference, the volume, or the surface area.   Diameter (D): the distance across the sphere – double the radius.  Diameter is the length of a line through the center of the sphere: from one point on the outside of the sphere to a corresponding point directly across from it.  In other words, the greatest possible distance between two points on the sphere.  Circumference (C): the one-dimensional distance around the sphere at its widest point. In other words, the perimeter of a spherical cross section whose plane passes through the center of the sphere.  Volume (V): the three-dimensional space contained inside the sphere. It is the "space that the sphere takes up."   Surface Area (A): the two-dimensional area on the outside surface of the sphere. The amount of flat space that covers the outside of the sphere.  Pi (π): a constant that expresses the ratio of the circle's circumference to the circle's diameter. The first ten digits of Pi are always 3.141592653, although it is usually rounded to 3.14. You can use the diameter, circumference, volume, and surface area to calculate the radius of a sphere. You can also calculate each of these numbers if you know the length of the radius itself. Thus, in order to find the radius, try reversing the formulas for these components' calculations. Learn the formulas that use the radius to find diameter, circumference, volume, and surface area.   D = 2r. As with circles, the diameter of a sphere is twice the radius.  C = πD or 2πr. As with circles, the circumference of a sphere is equal to π times the diameter. Since the diameter is twice the radius, we can also say that the circumference is twice the radius times π.  V = (4/3)πr3. The volume of a sphere is the radius cubed (times itself twice), times π, times 4/3.   A = 4πr2. The surface area of a sphere is the radius squared (times itself), times π, times 4. Since the area of a circle is πr2, it can also be said that the surface area of a sphere is four times the area of the circle formed by its circumference.

Summary:
Identify the basic measurements of a sphere. Use various measurements to find the radius.