The basic formula
For a constant cross-section, material weight is cross-sectional area × length × density. For carbon steel, a common planning density is about 0.2836 lb/in³. Keep units consistent before multiplying.
Plate and flat bar
Area is width × thickness. A 4 in wide × 1/4 in flat bar has a 1.0 in² cross-section. Multiply by 12 in/ft and steel density to estimate lb/ft.
Square or rectangular tube
Calculate the outside rectangle, then subtract the hollow inside rectangle. For a 2 × 2 × .125 in tube, the simplified inside dimension is 1.75 × 1.75 in. Real corner radii make published mill weight slightly different.
Round tube
Use the area of the outside circle minus the inside circle. Inside diameter is approximately OD − 2 × wall thickness.
When the formula is not enough
For structural shapes such as channel and angle, supplier/AISC published lb/ft is usually better than a simplified geometry estimate because rolled sections include fillets and standardized geometry.