sorry i don't obsess over richard size enough to know the technical terms ¯\_(ツ)_/¯
It's got very little to do with richards, it's actually basic geometry. You know,
2*pi*r and
pi*r2? Using the commutative property of multiplication you can figure out that
2*pi*r is the same as
(2*r)*pi, and
2*r is the same as diameter, which is how wide a cylinder is. So you get
pi*diameter = circumference (girth), and use some basic algebra to get
circumference/pi = diameter.
And they said you'd never use this stuff in real life.