What Is the Density for Water? Values, Units, and Why It Matters
At 20 °C, pure liquid water has a density of about 0.9982 g/cm³, or 998.2 kg/m³. For everyday estimates, this is often rounded to 1 g/cm³, so one millilitre of water has an estimated mass of one gram.
Water’s density changes with temperature and phase, so include those conditions when a precise value matters. Liquid water is densest at about 4 °C, where its density is close to 1.000 g/cm³. Ice is less dense than liquid water, so it floats.
How water’s density changes
Density is mass divided by volume. As water warms, it expands. The same mass then takes up more space, and its density falls. As liquid water cools from room temperature toward 4 °C, it contracts and becomes denser. Below about 4 °C, it begins to expand as it cools further.
Approximate values for pure water at ordinary pressure are:
| Temperature and phase | Approximate density |
|---|---|
| 0 °C, liquid just above freezing | 0.9998 g/cm³ |
| 4 °C, liquid | 1.000 g/cm³ |
| 10 °C, liquid | 0.9997 g/cm³ |
| 20 °C, liquid | 0.9982 g/cm³ |
| 25 °C, liquid | 0.9970 g/cm³ |
| 50 °C, liquid | 0.9880 g/cm³ |
| 100 °C, liquid at its boiling point near standard atmospheric pressure | 0.958 g/cm³ |
| 0 °C, ice | 0.917 g/cm³ |
These are rounded reference values. Exact density depends on temperature and pressure. For liquid water near boiling, pressure also affects the value. The USGS Water Science School explains how temperature affects water’s density and describes its density maximum.
The difference between water at 4 °C and 20 °C is small, so 1 g/cm³ works for many rough calculations. The difference grows at higher temperatures. A given volume of hot water has less mass than the same volume of cooler water.
As water freezes, its molecules form an open structure that takes up more space than the liquid arrangement. This gives ice a lower density, so it floats. Lakes generally freeze first at the surface. Depending on local conditions, the water beneath the ice can remain unfrozen.
Units and conditions
Density can be written in several equivalent units:
- 1 g/cm³ = 1 g/mL
- 1 g/cm³ = 1000 kg/m³
The units change how the number is expressed, not the physical quantity.
Specific gravity is a related, unitless ratio. It compares a substance’s density with the density of a reference substance, commonly water. The value depends on the reference conditions, so specific gravity should not be treated as a density with units.
Temperature matters when reporting water density. “The density of water is 1 g/cm³” is a convenient approximation, not a complete specification. For precise work, give the temperature and, where relevant, the pressure and phase. The IAPWS release on the density of ordinary water substance gives a formal method for calculating water density under specified conditions.
One worked example
Suppose you have 250 mL of pure liquid water at 20 °C and want to estimate its mass.
At that temperature, water’s density is about 0.9982 g/mL. Use the relationship:
mass = density × volume
So:
0.9982 g/mL × 250 mL = 249.55 g
The sample’s mass is about 249.6 g. Using the rounded density of 1 g/mL gives 250 g, a difference of about 0.45 g. That approximation works for many everyday estimates. A lab calculation may need a more precise density value and a controlled temperature.
Key Takeaways
- Pure liquid water has a density of about 0.9982 g/cm³ at 20 °C.
- Water is densest near 4 °C, at about 1.000 g/cm³.
- Ice at 0 °C has a density of about 0.917 g/cm³, so it floats in liquid water.
- 1 g/cm³ = 1 g/mL = 1000 kg/m³.
- State the temperature and phase when precision matters.
