Pipe & Tube Measurement; Understanding the differences and proper terminology

With Mark O'Neill



Did you know that there are numerous differences between pipe (circular hollow section – CHS) and rectangular hollow section (RHS)/square hollow section (SHS)?

Aside from the obvious - one being round and the other being square or rectangle - the main difference is in the terminology used to describe the sizing of each. This is commonly misunderstood and becomes confusing when trying to choose the correct materials for your project. These differences will be explained below, but before we look at the differences, let’s look at the similarities. Both pipe and RHS/SHS have two important measurements; the Outside Diameter (O.D.) and the wall thickness.

In the steel industry, the most important difference between pipe and RHS/SHS is the terminology used when describing the size of each material. The O.D. is used in conjunction with the wall thickness when describing RHS/SHS, whereas NB (nominal bore) is used for describing the size of pipe. This is often confusing to customers as they use the O.D. to describe pipe size but the steel industry uses nominal bore. By using the below table, we can translate O.D. sizes of pipe into NB sizes.

Let’s begin with understanding the correct way to measure RHS/SHS, as it is more straight forward than pipe. When RHS/SHS is measured, we use the total outside width and depth of the size of steel required, as the wall thickness increases on the inside of the hollow section therefore we are using the O.D. For example, if you require a piece of steel 50mm wide x 25mm deep you would order that size and choose the thickness of the wall you require, as the wall thickness will not change the outside size of the steel. See below for a table demonstrating the different sizes of RHS and SHS. The other option to choose when ordering RHS/SHS is whether you need the steel galvanized or painted.

Now let’s look at the correct way to measure pipe/CHS, remembering from above that we will be using nominal bore to describe its measurements - although if you have a knowledgeable steel salesman you will be able to use O.D. Like RHS/SHS, the wall thickness gets thicker on the inside of the pipe, but it will have differing inside diameter measurements. For example; a 50 NB medium wall pipe (medium wall being 3.6mm) will have an O.D. of 60.3mm. This can be calculated using the table below.




I hope this has explained the differences between pipe and tube measurements and has enabled you to gain a better understanding about the different terminology that is commonly used when referring to CHS and RHS/SHS. If you would like any further information, feel free to contact myself via moneill@dalbyruralsupplies.com.au or chat with one of our team by phoning (07) 46600400 or visiting us in store.