Tubs Parts supplies pipes and pipe fittings in inches, because this is the standard used by the most spa manufacturers. You cannot, however, just measure your pipe in inches to know the right size. That would be too easy. The standards were originally based on the internal diameter of the pipe, but they have evolved over time as new materials allowed for thinner pipe walls, and now they're just... well, standards.
Here are the outside diameters of each pipe size, in millimeters: