Pipe Measuring Guide

Please use these notes to help you to order the correct size pipe and fittings.  We cannot accept responsibility for you ordering the incorrect size for your needs.

Our pipe sizes shown here are for the Pressure Pipe we offer, and these can be different from standard domestic plumbing pipe from a hardware store.  We offer these in Metric (mm) and Imperial (inch) sizes. 

We will often refer to the O.D. (outer diameter) and I.D (inner diameter).  When measuring pipe we use the Outer diameter of the pipe, which will fit into a 'socket' fitting, which could be an elbow or a valve for example.  Some fittings are shown as a 'spigot' fitting, which means the fitting is the same size as pipe, and therefore will fit into a socket fitting.

 Schedule 40 vs Schedule 80 PVC

If you've been shopping around for PVC for American-made items such as hot tubs, you may have heard the term “schedule”. Despite its deceiving title, schedule doesn't have anything to do with time. A PVC pipe’s schedule has to do with the thickness of its walls. Maybe you've seen that schedule 80 pipe is slightly more costly than schedule 40. 
Though the outside diameter of a schedule 80 pipe and a schedule 40 pipe are the same, 80 pipe has thicker walls. This standard of measuring pipe came from a need to have a universal system for referring to PVC. Since different wall thicknesses are beneficial in different situations, the ASTM (American Society for Testing and Materials) came up with the schedule 40 and 80 system for classifying the two common types. 
The main differences between Schedule 40 (Sch 40) and Schedule 80 (Sch 80) are:
  • • Water Pressure Rating
  • • Sizing & Diameter (Wall Thickness)
  • • Color
  • • Application & Use


Imperial Pipe Sizes 

 Pipe Size  Outer Diameter
3/8 Inch  17.1mm
1/2 Inch 21.4mm
3/4 Inch  26.5mm
1 Inch  33.6mm
1 1/4 Inch  42.2mm
1 1/2 Inch  48.3mm
2 Inch 60.3mm
2 1/2 Inch  75.2mm
3 Inch 88.9mm
4 Inch  114.3mm
5 Inch 140 mm
6 Inch  168.3mm


 Metric Pipe Sizes 

Metric sizes are easier than imperial sizes.  The sizes indicated in this system are the actual dimensions of the pipe or fittings.  For example, 50mm pipe measures 50mm on the outer diameter. A 50mm socket will measure 50mm on the Inner Diameter.  A 50mm spigot fitting will measure 50mm on the outer diameter.


Threaded Fittings

Although we offer threaded fittings for 'Metric sizes' generally these fittings will be metric on the plain side (spigot or socket) and the thread will usually be Imperial BSP (British Standard Pipe).  A threaded fitting can be Male (screwing into a female thread) or Female (allowing a male thread to be screwed into it).  Please see 

Here is an example of a fitting with a Male thread

Here is an example of a fitting with a Female thread


BSP Thread Sizes

 Thread Size  O.D. in mm Threads per Inch Pitch in mm
1/8 Inch 9.7 28 .9
1/4 Inch 13.16 19 1.34
3/8 Inch  16.66 19 1.34
1/2 Inch  21 14 1.81
5/8 Inch 23 14 1.81
3/4 Inch 26.44 14 1.81
1 Inch 33.25 11 2.31
1 1/4 Inch  41.91 11 2.31
1 1/2 Inch  47.80 11 2.31
2 Inch 59.61 11 2.31
2 1/2 Inch  75.18 11 2.31
3 Inch 87.88 11 2.31
4 Inch 113 11 2.31
5 Inch 138.43 11 2.31
6 Inch  163.83 11 2.31