Density is mass per unit volume. The SI unit is kilograms per cubic metre, kg/m³, but grams per cubic centimetre and kilograms per litre are often more convenient, and they're the same thing: 1 g/cm³ equals 1 kg/L equals 1,000 kg/m³. The kilogram was originally defined, in the 1790s, as the mass of one litre of water, which is why water's density is so close to 1 kg/L. That link was broken when the kilogram was given a fixed physical definition, but the round number survived.
Where you meet it
Density matters whenever something is sold or measured by volume but limited by weight, or the other way round. Fuel is sold by the litre but a vehicle's payload and a boat's balance depend on its mass. Builders' merchants sell sand, gravel and concrete by the cubic metre or by the bulk bag, while a van or trailer is limited by weight. And many recipes and nutrition labels switch between millilitres and grams for liquids that aren't water.
Why the answer depends on temperature
Most materials expand as they warm, so their density falls. For water the change is small at everyday temperatures. For fuels it's larger, which is why fuel is traded commercially at a reference temperature. Loose materials add another variable: dry sand and wet sand have different densities, and so do compacted and loose gravel. When converting, use a density that matches the material's actual condition, and treat published figures as approximate.
Worked conversions
Diesel has a density of roughly 0.83 to 0.85 kilograms per litre, so a 50-litre tank of diesel weighs about 42 kilograms. Petrol is lighter, around 0.72 to 0.77 kg/L, so the same tank of petrol weighs about 37 kilograms. Concrete has a density of around 2,400 kg/m³, so a cubic metre weighs about 2.4 tonnes, more than many small vans can legally carry. Dry building sand is typically around 1,600 kg/m³, so a tonne of sand fills only about 0.6 cubic metres. That's why a "one-tonne" bulk bag isn't anything like a cubic metre in volume.
When precision matters, such as for loading limits, check the supplier's stated density and add a margin, because damp material can be noticeably heavier than the dry figure suggests.
Density in the kitchen
Density explains why a litre of flour weighs far less than a litre of water, and why some recipes give liquids in grams. Milk is about 3% denser than water, so 500 ml weighs about 515 grams. Honey and golden syrup are much denser, around 1.4 kg per litre, so measuring them by weight on digital scales is far more accurate than using a spoon or jug. Oils are lighter than water, typically about 0.91 to 0.92 kg per litre, which is why oil floats on top of a vinaigrette.
Checking your answer
Water is the reference: about 1 kg per litre, or 1,000 kg per cubic metre. Almost everything you'll convert is within a factor of about 20 of that. Most woods are lighter than water, many plastics are close to it, stone and concrete are two to three times heavier, and steel is about eight times heavier at 7,850 kg/m³. A result far outside that range usually means grams per cubic centimetre were mixed up with kilograms per cubic metre, a factor of 1,000.
One more worked example
A garden pond liner holding 2 cubic metres of water contains 2,000 litres, which weighs about 2 tonnes. That's why ponds and hot tubs on raised decks or roof terraces need structural checks: a 1,500-litre hot tub full of water and people can put several tonnes on a small area. Converting volume to mass first, using density, is the quickest way to see whether a load is something to worry about.