Litres are most commonly used for items (such as fluids and solids that can be poured) which are measured by the capacity or size of their container, whereas cubic metres (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The litre is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water.

One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 4 °C. Similarly: 1 millilitre of water has a mass of about 1 g; 1000 litres of water has a mass of about 1000 kg . This relationship holds because the gram was originally defined as the mass of 1 mL of water; however, this definition was abandoned in 1799 because the density of water changes with temperature and, very slightly, with pressure.

First time I heard about solids measured in litres. But, even if it's how Billzilla says, how does that work? I mean, for the customer a litre lacks meaning, you're not gonna mix it with water or have it in any other liquid form...

Well yeah, but for that matter why don't they just use a cubic unit, like cm3. Maybe because of that mix of systems that the Internet tells me exists in the UK?