The ultimate guide to metric and imperial tube, pipe and clamp sizes

Here we will explain all about tube and pipe size standards and terminology for both metric and imperial, and show you the various sizes of tube clamps we have available for both. 

NPS, NB, OD, DN… What does it all mean?!

Tube and pipe sizes can seem confusing, especially with the added complication of offering measurements in both metric and imperial, so let’s start with the basics and a glossary:

Pipes carry fluids and gases INSIDE them. So they are usually measured by their internal Diameter. (ID). 

Tubes are structural; they generally attach things on the OUTSIDE of them. They are usually measured by their Outside Diameter (OD)

Nominal Pipe size (NPS) is a USA standard which is an APPROXIMATE internal dimension of the pipe. It is approximate because the internal dimension of the pipe will change depending on the Schedule (wall thickness) of the pipe. . The outside dimension of the pipe always stays the same, this is why “pipe” in the USA and “tube” elsewhere are interchangeable sizes.

Nominal Bore (NB)  Same as Nominal Pipe Size

Diameter Nominal (DN)  Same as Nominal Pipe Size

OD  Outside Diameter

ID Internal Diameter

Schedule  A US standard which measures the wall thickness of the pipe. As the wall thickness increases, the internal diameter gets smaller. The outside diameter stays the same. 

Here at Domino Clamps, we use tubes as structural objects, so we refer to tubes and the clamps which attach them by the outside dimension of the pipe in mm. When using imperial equivalents, we will refer to them by the NPS, NB or DN measurement. (The approximate internal dimension).

What sizes of tube clamps can be bolted to Domino Clamps?

We now stock two different styles of clamp for pipe and tube. 

• Firstly the Interclamp range of cast iron (“key clamp”) tube clamps, available for 34mm, 42mm & 48mm outside diameter pipe and tube, in a range of orientations. 

• And Secondly our Unistrut range of “saddle clamp” -style pipe clamps, available in a range of sizes from 21mm - 114mm: outside diameter pipe and tube, held either vertically or horizontally.

For more information on what clamps are available in which size and for which orientations, please refer to our Ultimate guide to attaching metal tube to a shipping container 

 

The tube clamp specification says 48.3mm, why does it say 48mm?

The tube clamp sizes have been rounded to the nearest whole number for the sake of simplicity. Many other companies do this with their products - so if you see a 34mm OD tube, don’t worry! -that it is in fact 33.7mm, and it will fit in a tube clamp marked for 33.7mm tube.  Be aware however that there IS a difference between 48mm tube and 50mm tube. It is unlikely that any supplier would round up 48.3mm to 50mm, so this is likely to be a different grade of steel tube altogether. A 50mm outside diameter tube will not fit in our Interclamp range of 48mm tube clamps, but it will fit in size E Unistrut pipe clamps. 

Which size tube or pipe is designed to work with each style clamp? 

 

Interclamp clamps

The Interclamp tube clamps are made to suit steel tubing to BS EN 10255 (ISO 65), medium to heavy gauge, or equivalent sizes of tubing in other metallic materials such as aluminium,. Tubing of other specifications can be used, providing the steel is compatible with BS EN 10255 (ISO 65) and the wall thickness is not less than 3.2mm. The steel tube used in the UK and most of Europe has a wall thickness close to that of Standard or Schedule 40 pipe in the USA.

This style clamp relies on a snug fit, meaning that 48mm tube clamps won't usually accept 50mm diameter ERW steel tubes as they are just a fraction too large. They also tend to be too thin walled as well, so we recommend using the correct size; standard scaffold tube in the UK, Standard or Schedule 40 pipe in the USA. 

Unistrut pipe clamps

The Unistrut range of pipe clamps are a little more forgiving of variations in the outside diameter of the pipe, so for instance the 1 ½” NB size will clamp tube with an OD of 50mm. Also, because this style clamp does not rely on a grub screw to hold the pipe, it is better suited to pipe with a wall thickness less than 3.2mm, and for pipe in weaker materials such as Acrylic or PVC.

Can I buy the tube clamps directly from a US stockist?

Unfortunately, not at the moment. We haven’t yet found a US stockist of tube clamps which will fit our Domino Clamps. For the moment, you can see all our tube clamps in the shop, which should show you prices in US dollars if you are in the States.

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