Density is the property that turns geometry into weight, and it arrives in whichever unit the discipline prefers: kg/m³ from SI material tables, g/cm³ from chemistry and alloy datasheets (numerically the same as specific gravity referenced to water, which makes it the quickest sanity check), lb/ft³ from US construction and soils, lb/gal from drilling fluids, lb/in³ from machining stock tables, and slug/ft³ where US fluid dynamics wants consistent mass units. The anchors: water near 1,000 kg/m³ = 1 g/cm³ ≈ 62.428 lb/ft³ (exactly 1,000 only around 4 °C — 62.4 is the customary round figure), structural steel 7,850 ≈ 490 lb/ft³, aluminum 2,700 ≈ 169, normal-weight concrete about 2,400 ≈ 150.
Weight takeoffs are density arithmetic: weight = cross-section area × length × density × quantity, with the accuracy living in which density you pick. Alloy-specific datasheet values beat generic ones — carbon steel at 7,850 kg/m³ per the ASTM A36 datasheet, stainless 304 at the ASM Handbook’s 7,930 rather than the commonly rounded 8,000, lead at 11,340 per the CRC Handbook. The shop rule for mild steel plate falls straight out: 7.85 kg per square metre per millimetre of thickness.
In structural tables ρ serves weights and seismic mass, and non-metals print ranges rather than single values — concrete 2,240–2,400 kg/m³, softwoods 350–550 and hardwoods 550–800 — because grade, moisture, and formulation genuinely move the number; mill tolerance puts actual bar and plate weight within a few percent of the computed figure even with the right density.