Guide

Density in engineering and water quality

Density links mass and volume and decides whether a body floats, how much a liquid weighs and how layers arrange themselves in a vessel. It is stated in kg/m³, g/cm³ or g/l.

Definition and units

Density is mass per volume. The SI unit is kilograms per cubic metre. Laboratory practice uses grams per cubic centimetre, water management grams per litre. Note that 1 g/cm³ = 1000 kg/m³, while 1 g/l = 1 kg/m³. So g/cm³ and g/l differ by a factor of 1000.

Water as the reference point

Water has a density of about 1000 kg/m³ at 4 °C, that is 1 g/cm³. This is why the kilogram was originally defined via one litre of water. Today it is the other way round: the kilogram is fixed by constants of nature. Above and below 4 °C water is slightly lighter, which is why ice floats on water.

Density and temperature

The density of liquids and gases depends on temperature, that of gases additionally on pressure. For gases the dependence is strong: air is about 1.2 kg/m³ at 20 °C and only about 0.95 kg/m³ at 100 °C. Density figures without a temperature, and for gases without a pressure, are therefore incomplete.

What density decides in practice

Whether a body floats depends on whether its average density is below that of the surrounding medium. In water management density serves as an approximation for the content of dissolved substances, because salt water is denser than fresh water. In engineering it determines buoyancy and the layering in tanks. For gases it explains why propane collects near the ground and helium rises.

Matching conversions

Matching tables

Unit family: Density

Sources