Volume of a sphere

Euclid, among others, tells us that the volume of a sphere is given by
V = (4/3)πr³. That’s nice, but have you ever tried to measure the radius of a basketball?

It’s a lot easier to measure the circumference (provided you have a flexible tape measure).

As a public service, I present the following good approximation: V = r³/60.

The precise divisor is 59.217626 (to 6 significant figures). Using 60 gives a result that’s about 1% too small, so if you’re after better accuracy, add 1%.

About that radius thing: there is an easy way to find the radius of a ball (volleyball, basketball, beach ball): put it on a floor, next to a wall, so that it touches both the floor and the wall. The radius is the distance from the floor to where the ball touches the wall (or from there to the floor).