Density & Specific Gravity of Water vs. Temperature

Density of Water vs Temperature

Water behaves differently from most liquids. Instead of becoming denser as it cools, it reaches a maximum density at 4 °C (1000 kg/m³). This unusual property is critical in climate science, oceanography, and engineering calculations.

• 0 °C to 4 °C: Density increases as the ice-like molecular structure breaks down, allowing molecules to pack closer.
• At 4 °C: Water reaches its maximum density of ≈1000 kg/m³.
• Above 4 °C: Thermal expansion dominates — density decreases as molecular motion increases.
• At 100 °C: Near boiling point, density falls to about 958 kg/m³.
• At Critical Point (374 °C): Water becomes a supercritical fluid with density ≈322 kg/m³.

What is Specific Gravity (SG)?

Specific Gravity (SG) is the ratio of a substance’s density to the density of a reference, usually water at 4 °C (1000 kg/m³). Mathematically:

SG = ρ_substance / ρ_water@4°C

For pure water, SG = 1.000 at 4 °C. At other temperatures, it is slightly lower —for example, 0.998 at 20 °C. This makes SG a simple, dimensionless way to compare the density of liquids and solids relative to water.