Maximum density of water (4 °C)
| Value | 999.975 kg/m³ |
| Status | Measured: ± 0.001 kg/m³ (0.000001 relative) |
| Source | Tanaka et al. (2001), CIPM-recommended formula for VSMOW |
| Categories | ThermodynamicMaterial Properties |
| kilogram per cubic meter | 999.975 kg/m³ |
| gram per cubic centimeter | 0.999975 g/cm³ |
| gram per milliliter | 0.999975 g/mL |
| kilogram per liter | 0.999975 kg/L |
| pound per cubic foot | 62.4264 lb/ft³ |
| pound per US gallon | 8.3451958 lb/gal |
Learning zone
Almost every liquid gets denser as it cools, right down to freezing. Water reverses near 3.98 °C: below that, hydrogen bonds start assembling the open tetrahedral structure of ice, and the liquid expands again. The peak density is 999.975 kg/m³ — the reason the kilogram was originally meant to be the mass of a litre of water, and the reason that definition was so nearly right.
The consequence is ecological. As a lake cools in autumn the surface water sinks until the whole body reaches 4 °C; colder water is then lighter and stays on top, freezing into a floating lid that insulates everything beneath. Lakes freeze from the top down, not the bottom up, and fish survive the winter. Ice itself is 917 kg/m³, about 8 % less dense than liquid water, which is why it floats and why frozen pipes burst.