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).