The sphere is a well-known shape to nearly everyone. It is the most common shape used for a ball. There are some exceptions (Rugby Football for instance) but not many.
The radius is referred to in the calculator but this is a rarely used measurement in practice. It is the distance from the centre of the sphere to its outer surface and that is next to impossible to actually measure. The diameter however is very easy to measure and with great accuracy. This is the preferred dimension for engineers.
In the world of sport, where the rules lay down the size of the ball to be used, the circumference is generally specified. This is because it is easy for a referee or umpire to carry a tape-measure and put around the ball, judging by eye where the greatest distance around is. The rules usually state what limits the circumference must fall between, giving a minimum and a maximum.
This is simply one-half of a sphere.
To find its volumedivide volume of sphere by 2Finding its surface area requires a little more work.
Its surface is made up of two parts. One is the curved part, which is one-half the surface of the complete sphere. The other part is the flat surface which results from cutting the sphere in two. This part is a circle having the same diameter as the sphere.
So, to find surface area of hemi-spheredivide surface area of sphere by 2
and add on
area of circle of same diameter as sphere