Thank you for your query. All of our materials are listed as the external measurements unless stated otherwise. You can calculate the internal by subtracting the wall thickness from the external measurement twice. So, the internal of this particular channel would be 19mm.
We hope this helps!
We are specialists in metals, mild steel and aluminium suppliers and were the first online retailer of metals. We also provide a professional cutting service. Just tell us your requirements and leave the rest to us.
If you would like to speak to one of our Customer Services team please call us on 01937 534318 and we will be happy to help.
Convenient delivery options. Very low costs , starting at just £6.00 & free for orders over £75.00 to England & Wales. Hassle free returns if needed.