pi (π), circles and spheres

pi – π

pi is a number that goes on forever. So it has every number pattern possible somewhere inside it. It can be used to work out area and volume of circles and spheres.

Usually we can use pi with just 2 decimal places:

pi = 3.14

I have memorised it to 7 places as this is what a calculator shows:

pi = 3.1415927

My Dad memorised it to 21 decimal places when he was 10:

pi = 3.141592653589793238462


Circumference of a circle:

2πr (r is the radius which you can see from my picture below)

Diameter is 2 times the radius so the circumference can also be πd

Area of a circle:

πr2 (r2 means radius squared which means radius times itself)

(this picture I drew using Inkscape but I had to save it as a png as the svg file could not be seen).


Surface area of a sphere:


Volume of a sphere:


(this picture was from wikipedia)