Confused by sizes in inches and millimeters?

Have you measured a pipe to replace it, but don't know how to convert from metric to imperial or vice versa? Don’t panic, even if it's a bit more complicated than it seems, we're here to help you avoid any potential mistakes.

1 - Converting measurements doesn't work

Pipes and fittings are listed in inches on our website, because the imperial system is the standard unit of measurement for the vast majority of spas manufactured worldwide.

However, you can't just convert inch sizes to millimetres on the basis of 1 inch = 25 mm. Unlike metric pipe sizes, the names of inch sizes are based on the interior diameter of the pipe. As materials have evolved, the pipe walls have become thinner to the point where the inside of a standard 2-inch pipe actually measures 2.1" (53 mm), and the outside measures 2.4" (60 mm). No dimension on a "2-inch pipe" corresponds to 2 inches any more (unless you have exactly 2 inches of a 2-inch pipe, but I digress).

Here are the main pipe sizes in inches, with their diameters in millimetres, and their closest equivalents in metric pipes. Lister below is the outside diameter of the male fittings, and therefore the inside diameter of the female fittings to choose in correspondence.

Inch pipe sizes	Diameter in mm	Closest metric equivalent
1/4"	9 mm
3/8"	14 mm
1/2"	21 mm	20 mm adapter
3/4"	27 mm	25 mm adapter
1"	33 mm	32 mm adapter
1,5"	48 mm	50 mm adapter
2"	60 mm	63 mm adapter
2,5"	73 mm	75 mm

Unless specified otherwise, all our pipes and fittings are made of ABS and comply with the "Schedule 40" or SCH 40 measurement standard, which defines the pipe wall thickness and therefore the diameter.

For a 1" male pipe (outer diameter 33 mm) for example, its inside diameter will correspond, according to SCH 40 standard, to 26 mm. This measure is important if it is not possible to use a female sleeve and you wish to connect a M+ coupler, which will fit inside the pipe.

However, this standard is not universal, which means that although we know the outside diameter of a pipe (male fitting), we do not necessarily know its inside diameter. As soon as possible, Tubs Parts will provide you with the inside and outside diameters of each pipe and fitting.

Generally speaking:

• For pipes smaller than 1", bonding to the inside surface (with a coupler) works best because the diameter deviations are relatively small.
• For 1" and larger pipes, glueing on the outside of the pipe is preferred if possible. Adhesives should be used on the inside surface of 1" pipes and no longer only if there are no other alternatives (e.g. pipe cut too short).