Image caption, The diameter is the whole distance across the circle, through its centre. The diameter is two radii. The radius is half of the diameter. The radius is the distance from the centre ...

The radius is the distance halfway across the circle, from the centre to the circumference. The radius is always half the length of the diameter. If you know the radius of this circle is 6cm ...

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 ...