Convert density

About the units

• Kilograms per cubic metre (kg/m³) – The kilogram per cubic metre is the SI coherent unit of density and a legal unit throughout the EU as a combination of the kilogram and the cubic metre. It is used for building materials, concrete, insulation, process liquids and air density in meteorology. Water is about 1,000 kilograms per cubic metre, but that figure is no longer a definition: the litre was defined in 1901 as the volume of one kilogram of water, and that definition was repealed in 1964.
• Grams per cubic centimetre (g/cm³) – Grams per cubic centimetre is built from the gram and the cubic centimetre, both decimal forms of SI units and legal throughout the EU. The unit comes from the old CGS system and is convenient in chemistry, materials science, mineralogy and metallurgy, because water then has a density of about 1. One gram per cubic centimetre is exactly 1,000 kilograms per cubic metre, and the same figure holds for kilograms per litre and tonnes per cubic metre. That factor of a thousand is the commonest slip here.
• Kilograms per litre (kg/l) – The kilogram per litre is built from two legal units and is therefore legal throughout the EU. Its figure matches grams per cubic centimetre and tonnes per cubic metre, and is a thousandth of that in kilograms per cubic metre. It suits liquids: Swedish industry tables give ethanol-free petrol 750 kilograms per cubic metre, or 0.75 kilograms per litre. The trap is water. The litre of 1901 was the volume of a kilogram of water, but that definition was repealed, and water peaks at about 0.99997 kilograms per litre.
• Pounds per cubic foot (lb/ft³) – Pounds per cubic foot is avoirdupois pounds per cubic foot. Since both the pound and the foot are exact figures the conversion is exact: one pound per cubic foot is 16.01846337396014 kilograms per cubic metre. The unit is used in American building and materials engineering for concrete, timber, insulation and soil, and in oil and gas; in the EU it is not a legal unit. Water is 62.43 pounds per cubic foot. The trap is pounds per gallon, nearly 7.5 times larger.
• Pounds per US gallon (lb/gal) – Pounds per gallon here means avoirdupois pounds per US gallon, 119.826427 kilograms per cubic metre or just under 0.12 kilograms per litre, and water comes to about 8.3. The unit belongs to American oil and gas drilling: US offshore rules check whether returning drilling fluid is within 0.2 pounds per gallon of the fluid going in, or 1.5 pounds per cubic foot. The trap is the gallon. On the imperial gallon the unit is 99.78 kilograms per cubic metre, and water weighs about ten pounds per gallon.