Dimensions of hot tub pipes with imperial measurements baffles most of us until we understand how they work. This is totally at odds with metric size pipes, and I'll explain how. Imperial measurements for hot tub pipes remain the 'standard' used globally.
Almost all hot tub pipe in the UK hot tub industry is based on the North American set of standard pipe sizes. It's based on 'Nominal Pipe Size' which refers to the nominal size of the pipe. This is a size that falls in between the inner and outer diameter of the pipe. The wall thickness for PVC hot tub pipe is what's known as 'Schedule 40'; this is what determines the actual diameter of hot tub pipe. The required 'Schedule' of PVC pipe has been determined by the temperature and pressure of water flowing through the pipe.
As an example: 2 inch hot tub pipe measures 2.375 inches (60.33mm) outside diameter.
In contrast, metric pipe sizes are the size they are labelled as. For example: 50mm pipe measures 50mm. Pipe like this clearly doesn't originate from North America, because they don't recognise or understand millimetres. It means hot tubs with metric pipe most likely came from Asia.