Plasson Rural Coupling / Joiner

Threaded Fitting Sizes - A Short Guide

NPT (National Pipe Taper - as USA standard) and BSPT (British Standard Pipe Tapered - a European standard) are the two most common plumbing & irrigation thread types we come across in Australia.
Both of these are tapered threads (meaning they are broader at the top of the thread and narrower at the insertion end), and they are imperial standards (specified in inches, or fractions of).

There is also a BSPP (British Pipe Thread - Parallel) standard.
These fittings have parallel threads (same diameter bottom to top), and it is also an imperial standard (specified in inches, or fractions of).
Where a fitting is not specifically identified as being BSPT, it is usually safe to assume it is a parallel thread (but not always).

Because almost all Australians prefer to speak metric these days, yet these are imperial standards, there is often more than one accepted metric "equivalent" to a particular imperial thread size.

The accepted conversion depends on the fitting material, manufacturer, or the plumber/irrigation tech you are talking to!

It is very important to remember that metric-to-imperial conversions are only accepted when talking about thread sizes (never pipe sizes).
There are both imperial & metric pipes, and they are NOT interchangeable (imperial or "rural" pipes are measured "ID" - or inside diameter, whereas metric pipes are measured "OD", or outer diameter).

This makes plumbing & irrigation with threaded fittings either loads of fun, or a massive headache - depending on your life outlook!

Below is a rough guide to imperial sizes and their accepted metric conversions