well what are we saying the handbreadth is, about 4 inches?
A hand's breadth is approximately 1/4 of a cubit -- some use 4 inchers and others use 4.5 inches) A span is approximately 1/2 of a cubit or about 9 inches.
So the inside diameter would be approximately 9 1/2 cubits. So mulitply 9.5 cubits by pi and approximately how many cubits do you have?