Density of Water? The density of a material is defined as its mass per unit volume. It’s a measurement of how tightly matter is packed together. The density of a substance can be explained as the relationship between the mass of the substance and volume it takes up. Water density changes with temperature and salinity. Density is measured as mass (g) per unit of volume (cm³). Water is densest at 3.98°C and is least dense at 0°C (freezing point).