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The density formula is simple: divide mass in kilograms by volume in cubic metres to get density in kg per cubic metre. Divide that figure by 1000 for g per cubic centimetre, which is also the specific gravity, since water has a density of 1000 kg per cubic metre. For a regular shape, work out volume from its dimensions; for an irregular shape, measure volume by water displacement.
Density is mass packed into a given volume. A 2.7 kg block with a volume of 0.001 cubic metres has a density of 2700 kg per cubic metre, which is the density of aluminium. The same mass in a larger volume gives a lower density, which is why a foam block feels light next to a brick of the same size.
The SI unit of density is kg per cubic metre, while g per cubic centimetre is common in chemistry and labs. One g per cubic centimetre equals 1000 kg per cubic metre, so water is 1 g per cubic centimetre and 1000 kg per cubic metre at the same time. Other units include g per mL, kg per litre and lb per cubic foot.
Common densities help you sense check a result: water 1000, ice 917, ethanol 789, olive oil 911, oak wood about 750, concrete 2400, glass 2500, aluminium 2700, steel 7850, copper 8960, lead 11340 and gold 19320, all in kg per cubic metre. Alloys and moisture vary these figures, so treat them as typical values.
Specific gravity is the density of a material divided by the density of water, so it has no units. Steel has a specific gravity of about 7.85, meaning it is 7.85 times denser than water, while aluminium is about 2.7, roughly one third of steel.
Enter the mass in kilograms and the volume in cubic metres, then read the density in both kg per cubic metre and g per cubic centimetre, plus the specific gravity. For a regular shape, work out volume from its dimensions. For an irregular shape, measure volume by water displacement.
Density changes with temperature, because most materials expand when heated and contract when cooled. The effect is small for solids and liquids but large for gases, where doubling pressure roughly doubles density. Water is unusual: it is densest at about 4 degrees Celsius, then expands as it freezes, which is why ice floats.
Buoyancy follows density directly: an object floats if its density is below the fluid it sits in, and sinks if it is above. Oil floats on water because olive oil at 0.91 g per cubic centimetre is less dense than water at 1.0, while a steel block at 7.85 sinks fast.
An aluminium bracket of 2.7 kg in 0.001 cubic metres gives 2700 kg per cubic metre, 2.7 g per cubic centimetre and a specific gravity of 2.7. A rock of 100 g displacing 25 mL of water has a volume of 25 cubic centimetres, giving 4 g per cubic centimetre, or 4000 kg per cubic metre.
Density equals mass divided by volume. Weigh the object, find its volume in matching units, and divide mass by volume.
Water has a density of 1000 kg per cubic metre, which is 1 g per cubic centimetre or 1 g per mL. It is densest at about 4 degrees Celsius.
The SI unit is kilograms per cubic metre, written kg per cubic metre. Other common units are g per cubic centimetre, g per mL, lb per cubic foot and lb per gallon.
Specific gravity is the density of a material divided by the density of water, so it has no units. Steel has a specific gravity of about 7.85, meaning it is 7.85 times denser than water.
Mild steel has a density of about 7850 kg per cubic metre, or 7.85 g per cubic centimetre, with small variations by alloy.
Aluminium has a density of about 2700 kg per cubic metre, or 2.7 g per cubic centimetre, which is roughly one third of the density of steel.
Use water displacement: fill a measuring container, note the level, submerge the object, and read the rise. One millilitre of rise equals one cubic centimetre of volume.
Ice has a density of about 917 kg per cubic metre, less than liquid water at 1000, because water expands when it freezes. That is why ice forms on the surface of lakes rather than sinking.
Density helps identify materials, especially metals, but it is not proof on its own. Alloys, porosity and temperature all shift the figure, so combine it with other tests for a firm identification.