kilogram per cubic meter

The kilogram per cubic metre is sized for gases and bulk materials, and its most underrated number belongs to air. At 1.2 kg/m³, the air filling an ordinary 50 cubic metre living room weighs about 60 kg, as much as an adult, a fact almost nobody's intuition contains because nobody ever has to lift it. That density is what fans push, wind loads press with and aerodynamic drag acts on, and it thins with altitude: an airliner at cruise flies through air of roughly 0.4 kg/m³, one reason flying high saves fuel.

Matter spans about five orders of magnitude in this unit. Aerographene, the lightest solid made, is around 0.16 kg/m³, less than the air it displaces until you count the air soaked into its pores; osmium, the densest element, is 22 590. Between them sit dry timber at a few hundred, water at 1000, rock near 2700, steel at 7850 and gold at 19 300.

The working trap is the factor of a thousand against g/cm³. A chemistry table reads 7.85 for steel where an engineering table reads 7850, the same measurement in units a thousand apart, and dropping or doubling that factor is the classic density error. Buoyancy, Reynolds number and mass-from-volume calculations all want this unit, the coherent one, so a value imported from a chemistry table must be multiplied by 1000 on the way in.