How much weight can mild steel square tubing support?
The tubing that you suggested 1800mm x 25mm x 25mm with wall thickness of 3mm can be converted to an approximate dimension of 72" long, 1"x1" square outside size and a wall thickness of 0.118". The deflection calculator at this site suggests that such tubing supported at the very ends would experience a deflection of almost a half an inch with a 100 pound load applied right at the center of its length.



I would not consider this piece of tubing to be at all suitable for supporting a table or desk that was 1800mm from end to end and only supported at the ends. You would be far better off to use rectangular tubing that was 25mm x 50mm with the 50mm deployed in the vertical direction. You could also consider the use of 50mm x 50mm square tubing with a wall thickness of 5mm and achieve much better results.