The circumference and areas of circles are calculated with pi.
π, that is.
π is a number with infinite digits after its decimal point in a pattern which never repeats. It is roughly 3.14159, or *π to a million digits*.
It is used in two different formulae, 2πr and πr², for the circle’s circumference and area respectively. r in these formulae refers to the radius of the circle.
This is all quite simple.
It gets more complicated as you go into 3D shapes – surface area is the sum of the areas of all the shapes faces but volume has different equations for different shapes. The simplest is that of prisms, such as cuboids. As they are the same all they way through, it can simply be calculated by multiplying the area of the face which extends the whole way through by the length of the prism. Eg: This cylinder:
π x 9² = 254.5
245.5 x 17 = 4329.98cm³