Why wall thickness changes everything
Square tube is hollow. A small increase in wall thickness adds material around all four sides, so pounds per foot can rise much faster than many people expect.
Calculation method
Subtract the hollow inside square from the outside square, then multiply the remaining area by 12 inches and material density.
lb/ft ≈ [OD² − (OD − 2t)²] × 12 × density
Example
For 2 × 2 × .125 in carbon-steel tube, the simplified square-corner model produces a little over 3 lb/ft. Supplier tables can be slightly different because real tubing has radiused corners and manufacturing tolerances.
Use published weight when it matters
For freight, engineered structures or purchase verification, the supplier or mill published weight is the better final source.