General Information

What is a threaded fitting?

This is a method of joining pipe systems together.


The sizes of pipes, fittings, valves and flanges are generally referred to by their nominal sizes (Nominal Pipe Size or NPS).

This can either be in metric units (DN - Diametre Nomimal - mm) or imperial units (inches).

A 1” threaded fitting is also known as a 25mm threaded fitting.

Note: the naming convention refers to the inside diameter (ID) of the fitting.

How to size a threaded fitting

The outside diameter for a 1” BSP threaded fitting is not 1” or 25mm but approximately 33mm. Once you have the Outside Diameter (OD) measurement for your fitting, consult the table below for its nominal size.

Nominal Pipe Size - NPSOutside Diameter
DN  (mm)InchmmInch
61/8”9.728 mm0.383
81/4”13.157 mm0.518
103/8”16.662 mm0.656
151/2”20.995 mm0.825
203/4"26.441 mm1.041
251”33.249 mm1.309
321-1/4”41.91 mm1.650
401-1/2”47.803 mm1.882
502”59.614 mm2.347
652-1/2”75.184 mm2.960
803”87.884 mm3.460
1004”113.03 mm4.450
1255”138.43 mm5.450
1506”163.83 mm6.450

Different thread types

Two thread types are distinguished - BSP and NPT. The difference has more to do with the country of origin/ your location than the application. BSP threads are common across UK, Europe, Australia, New Zealand, South Africa, and most other countries. NPT are the standard pipe threads in USA and Canada.

Summarised differences for the thread types

Thread AcronymNameSeriesPeak & ValleysThread AngleAlso referred to as…
BSPBritish Standard PipeStandard & TaperedRounded55°Whitworth Thread (after designer Joseph Whitworth)
NPTNational Pipe ThreadStandard & TaperedFlat60°Sellers Thread (William Sellers established US Standard)
Threads can be either tapered or parallel. In the case of BSP threads:
  • Parallel (BSPP)
  • Tapered (BSPT)

Tapered vs Parallel

Tapered threads are utilised because a taper thread can be pulled tightly to create a fluid or air-tight seal. The male threads seal against the female thread.

This will not be the case for parallel or straight threads. These have been designed to hold pieces together but not to create a seal. In such cases a washer or O-ring is needed to affect a seal.

Common Questions

What threads are commonly used in Australia?

BSP thread is common in Australia and the commonwealth countries.

How to size a BSP threaded fitting?

Measure the outside diameter of your male threaded fitting and consult the table above