Water is densest at 4°C, not at freezing
Most liquids get steadily denser as they cool. Water does not. It peaks at 4°C and gets lighter on both sides — warmer water expands the usual way, and colder water expands too as hydrogen bonds open into an ice-like structure. That reversal is the whole story of water density.
The (T − 4)² term is why density falls away from the 4°C peak in either direction. It is accurate near room temperature; the full curve is tabulated below.
Density at common temperatures
| Temperature | Density (kg/m³) | g/mL |
|---|---|---|
| 0 °C (liquid) | 999.84 | 0.99984 |
| 4 °C | 999.97 | 1.00000 |
| 20 °C (room) | 998.21 | 0.99821 |
| 25 °C | 997.05 | 0.99705 |
| 40 °C | 992.25 | 0.99225 |
| 60 °C | 983.20 | 0.98320 |
| 100 °C (boiling) | 958.40 | 0.95840 |
The everyday "water is 1 g/mL" holds only near 4°C. By boiling it has dropped over 4 percent. As a rule of thumb, each 1°C near room temperature shifts density by about 0.2 kg/m³.
The anomaly is why ice floats
When water freezes it does not keep shrinking — it locks into an open lattice and expands, so ice is lighter than the liquid it came from and rises to the top.
Ice is about 8% lighter than liquid water, so it floats — and lakes freeze top-down, insulating the life below.
Common questions
What is the density of water at 4 degrees C?
About 999.97 kg/m3, or essentially 1.000 g/mL. This is the maximum density of pure water and the reason 4 degrees C is used as a reference point in physics and chemistry.
Why is water densest at 4 degrees C and not at freezing?
Below 4 degrees C, hydrogen bonds start arranging the molecules into an open, ice-like lattice that takes up more space, so the water expands and gets lighter as it cools further. Above 4 degrees C the usual thermal expansion takes over. Both effects pull density down away from the 4 degree peak.
Why does ice float on water?
Because that open lattice makes solid ice about 917 kg/m3, roughly 8 percent lighter than liquid water at 1000 kg/m3. Being less dense, it floats. This is why lakes freeze from the top down instead of solid, letting fish survive winter.
