Confused by sizes in inches and millimeters?

Confused by sizes in inches and millimeters? - Click to enlarge

You want to buy a pipe from measurements taken in millimeters, but you don't know how to convert from metric to imperial? Don't panic, here are some explanations to help you avoid mistakes.

1 - Converting measurements doesn't work

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

Be careful, though: 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 "2-inch pipe" corresponds to 2 inches any more.

Here are the main pipe sizes in inches, with their diameters in millimeters, and their closest equivalents in metric pipes.

Inch pipe sizes Diameter in mm Closest metric equivalent
1/4" 9 mm
3/8" 17 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
Remember, this is the outside diameter of the male fittings, and therefore the inside diameter of the female fittings to choose in correspondence (and vice versa), of course.

2 - Special connections

Unless otherwise specified, 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 aM+ 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, 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, gluing 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).