SWG conversion is one of the most common questions we receive about sizes. The metal industry does like to take its own time catching up with modern parlance and still often uses this quaint unit of measurement.

Round tube inside diamaters

To get the measurement of the inside diameter of a tube, you need the outer diameter and the thickness of the wall measurements. However, it is sometimes forgotten that the wall thickness has to be multiplied by two.
Many customers do get tripped up by this so let’s give a quick example of how to do it correctly:

  • We have a round tube with an outer diameter of 30mm and a wall thickness of 3mm.
  • Multiply the wall thickness by 2 to give us 6mm.
  • The outer diameter of 30mm minus the 6mm then gives us a 24mm internal diameter.

Telescoping tube

Which brings us to one our most infamous customer enquiries… “I’m trying to fit tube A into Tube B, which in turn, will fit into Tube C….”. Day turns into night as we frantically leaf through our stock sizes before calling down to the warehouse to physically try it out. We usually do find a solution but if you want to size this up yourself, the main thing to remember is that a 1 inch bar will not fit into a 1 inch hole. Not without some severe rubbing down. Also, tolerance factors can trip up the whole scenario.

You may find some tube sizes which fit beautifully together with minimal amount of clearance to make it nice and snug. However, there are always tolerance factors which may throw the figures out. We are always happy to help to find the right sizes for you and we do suggest you ask us to check current stock to test the sizing solution.

Imperial to metric

All our online sizes can be calculated into metric if required during the production selection stage. To calculate inches to mm, you simply multiply by 25.4. For example, 3 inches multiplied by 25.4 would give us 76.2mm. To calculate the other way, you would simply divide.