1 g/cm³ equals exactly 1000 kg/m³. This is one of the most useful density identities in science: the density of water at 4 °C is 1.000 g/cm³, which is exactly 1000 kg/m³. Grams per cubic centimetre is common in chemistry and materials science, while kg/m³ is the SI unit used in physics and engineering.

Pure water reaches its maximum density of 1.000 g/cm³ (1000 kg/m³, 62.43 lb/ft³, 8.345 lb/gal) at 3.98 °C. At 20 °C it is 0.9982 g/cm³. Salt water is denser — seawater averages about 1025 kg/m³ — and ice is less dense at 917 kg/m³, which is why it floats.

Density is a physical quantity with units (kg/m³, g/cm³, etc.). Specific gravity is a dimensionless ratio — the density of a substance divided by the density of water at 4 °C (1000 kg/m³). A specific gravity of 2.5 means the substance is 2.5 times denser than water, which equals 2500 kg/m³ or 2.5 g/cm³.

1 lb/gal (US) equals approximately 119.83 kg/m³. To convert, multiply lb/gal by 119.83. For example, water at 8.345 lb/gal equals 8.345 × 119.83 ≈ 1000 kg/m³. Note that UK (imperial) gallons are larger than US gallons, so 1 lb/gal (UK) ≈ 99.78 kg/m³.