Every circle has a radius (r) — the distance from the centre to the edge — and a diameter (d = 2r) — the distance all the way across.
Circumference (the perimeter of a circle) = 2πr = πd. This is the distance around the outside.
Area = πr². This is the space inside.
π (pi) is approximately 3.14159. It's the ratio of any circle's circumference to its diameter — always the same for every circle ever.
Remember
Area uses r² (radius squared). Circumference uses r (radius, not squared). A common mistake is using the area formula when you want circumference and vice versa.
Real World
A circular pizza — the circumference tells you how long the crust is. The area tells you how much topping you need. Both use the same radius.
Feeling ready?
Put this concept to work with a practice workout.