Pipe & Fitting Diameters / Conversions

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 pressure pipe            -  21.4mm               
3/4" / 20mm pressure pipe            -  26.8mm
1" / 25mm pressure pipe               -  33.5mm
32mm pressure pipe.                    -  42.3mm
1.5" / 40mm pressure pipe            -  48.4mm
2" / 50mm pressure pipe               -  60.5mm
2.5" pressure pipe                         -  73mm
65mm pressure pipe                     -  75.4mm



These measurements are of the INTERNAL diameter of clear vinyl TUBING (CVT) 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.


Imperial to Metric Dimensions

Inches to millimetres
1"  =  25.4mm

Flow Rates 
US Gallon Per Minute to Litres Per Minute
1.00 US GPM  =  3.785 LPM