Trying to work out what size pipe or fitting you have? Wondering why measurements don't look right?
40mm pipe is not 40mm diameter!

Common Pipe, Hose, Tubing and Fittings diameters used on spas and pools

Pipe and Hose

These measurements are of the EXTERNAL diameter of the pipe or hose (and spigot fittings, or internal diameter of socket fittings):

1/2" / 15mm            -  21.4mm               
3/4" / 20mm            -  26.8mm
1" / 25mm                 -  33.5mm
1.5" / 40mm             -  48.4mm
2" / 50mm                 -  60.5mm
2.5" / 65mm             -  73mm

Tubing

These measurements are of the INTERNAL diameter of TUBING only:

3/8" / 9mm               -  9mm
3/4" / 19mm             -  19mm

External diameter of the barb fittings used with tubing is generally slightly larger than the bore of the tubing to ensure a tight fit.

The internal diameter of pipe and hose varies with class ratings and manufacture. The external diameter of tubing varies with wall thickness and type of tubing used.

Why the confusion? Depending on the class (wall thickness) of the pipe, measurements are all over the place. Originally pipe sizes were based on internal diameters. All fittings were made to glue over the outside of the pipe. As a result, the outside diameter became the critical measurement for pipe sizing. Over time different grades of pipe (and consequently wall thickness) became more popular, so even measuring the internal diameter is inaccurate. The original sizes used and requested have 'stuck' and are now causing confusion, particularly with consumers trying to match pipe & fitting sizes for repairs.