I measured the ball. But the cover was too small. So I bought a new ball. Ball was still too big and seemed to be not firm enough when inflated to the “proper size” of the string provided. So I put the ball in the cover and then tried to sit on it. It was much firmer somehow. I imagine the fabric keeps the ball from flattening as much because the fabric only stretches along the bias one way. If I sit on the circle, the ball does flatten out and not support as well. I’m sure there are physics at work here, but I couldn’t explain them. Quality fabric and I can remove the cover for washing without deflating the ball. That is the win for me. I haven’t washed it yet, but I’m sure I will get around to it. Lots of illness going around the office and I haven’t used it a full two weeks yet.