Pi numerically is 3.1416 and you know this already. A Pi tape is used to wrap around the circumference of a cylinder to arrive at the diameter. As has already been commented, they work well on irregular or distorted shapes and soft plastic, rubber, etc. When all else fails or you just don't have a tape, wrap a piece of sturdy tape around the outside, slice it with a knife, peal it off and measure it, then divide this by 3.1416. I wouldn't look for .0001"accuracy this way but you're in the ballpark close.