From the centre to the edge is the radius, from the edge to the edge all the way across is the diameter. It's simples. Look you do this, I gotta go out. Five pizza. Remember. Good boy. VO ...

The diameter is two radii. The radius is half of the diameter. The radius is the distance from the centre of the circle to the circumference, or the distance from the circumference of the circle ...

which means you can figure out the circumference of any circle if you know its diameter or radius (half the diameter) and pi, which is most commonly abbreviated as 3.14 — which is why Pi Day is ...