Water Density Calculator
Enter water temperature to find its density instantly. Switch to seawater mode to add salinity and pressure effects. Results update as you type and include kg/m3, g/cm3, and lb/ft3. The step-by-step panel shows exactly how the calculation was done, and a reference table lists standard density values across the full temperature range.
Why water density changes with temperature
Water molecules are held together by hydrogen bonds. As temperature rises, the molecules gain kinetic energy and move farther apart, reducing density. But there is an unusual twist: ice and very cold water are less dense than liquid water at about 4 C, because water molecules form a hexagonal lattice in ice that spaces them out more than the liquid state. This is why ice floats and why lakes freeze from the top down. At exactly 4 C (39.2 F), liquid water reaches its maximum density of 999.975 kg/m3. Above and below that temperature, density falls, creating a curve with its peak at 4 C.
How salinity and pressure affect density
Dissolved salts increase water density by adding mass without proportionally expanding volume. Average ocean water at 35 ppt salinity is roughly 25 kg/m3 denser than fresh water at the same temperature. This density difference drives thermohaline circulation: cold, salty water at the poles sinks and flows along the ocean floor toward the equator, while warmer, lighter water moves along the surface to replace it. Pressure also increases density slightly because water is mildly compressible. At 100 m depth (about 10 atm gauge), density increases by roughly 0.5 kg/m3. At ocean-trench depths of 11 km (about 1100 atm), the increase reaches around 50 kg/m3.
The Kell polynomial and Millero method
This calculator uses the Kell (1975) 5th-order rational polynomial for pure water density from 0 to 100 C at atmospheric pressure. The polynomial fits IAPWS-95 data to better than 0.001 kg/m3 across the full range. For seawater, a simplified form of the Millero et al. equation adds salinity-dependent coefficients (A, B, C terms) and a bulk-modulus pressure correction. The Millero approach underpins the UNESCO ocean standard and is accurate to about 0.01 kg/m3 in the oceanographic range of 0-40 C, 0-40 ppt and 0-1000 atm.
Practical applications of water density
Water density matters across many fields. In hydraulics and civil engineering, pipe-flow calculations depend on the density of the water at operating temperature. Aquaculture and fish hatcheries monitor density to infer dissolved oxygen. Brewers and winemakers track density changes via a hydrometer to measure sugar conversion during fermentation. Marine engineers design ships with stability margins that account for the density difference between ocean and river water (a ship floats higher in the Atlantic than in the Thames). Oceanographers use density profiles to map water masses, currents, and climate-relevant heat content in the deep ocean.
Water density at standard pressure (1 atm) by temperature
| Temperature (C) | Temperature (F) | Density (kg/m3) | Density (g/cm3) | Specific gravity |
|---|---|---|---|---|
| 0 | 32 | 999.841 | 0.999841 | 1.00000 |
| 4 | 39.2 | 999.975 | 0.999975 | 1.00013 |
| 10 | 50 | 999.700 | 0.999700 | 0.99972 |
| 15 | 59 | 999.099 | 0.999099 | 0.99912 |
| 20 | 68 | 998.204 | 0.998204 | 0.99823 |
| 25 | 77 | 997.047 | 0.997047 | 0.99707 |
| 30 | 86 | 995.652 | 0.995652 | 0.99568 |
| 40 | 104 | 992.215 | 0.992215 | 0.99224 |
| 50 | 122 | 988.041 | 0.988041 | 0.98807 |
| 60 | 140 | 983.195 | 0.983195 | 0.98321 |
| 70 | 158 | 977.763 | 0.977763 | 0.97779 |
| 80 | 176 | 971.800 | 0.971800 | 0.97183 |
| 90 | 194 | 965.341 | 0.965341 | 0.96537 |
| 100 | 212 | 958.405 | 0.958405 | 0.95843 |
Values calculated using the Kell (1975) polynomial for pure water. Seawater at 35 ppt is approximately 24-26 kg/m3 denser across the range.
Frequently asked questions
What is the density of water at room temperature?
At 20 C (68 F), pure water has a density of about 998.2 kg/m3, or 0.9982 g/cm3. This is close to but slightly below its maximum of 999.975 kg/m3 at 4 C. For most everyday calculations you can use 1000 kg/m3 (1 g/cm3) as a convenient round number with less than 0.2% error.
At what temperature is water densest?
Pure water reaches its maximum density at approximately 3.98 C (about 4 C or 39 F), where it is 999.975 kg/m3. Below that temperature, hydrogen-bond geometry expands the lattice and density decreases toward 917 kg/m3 for ice at 0 C. Above 4 C, increasing thermal agitation reduces density continuously up to the boiling point.
How does salinity change the density of seawater?
Each gram of dissolved salt per kilogram of water (1 ppt) adds roughly 0.8 kg/m3 to the density. Average seawater at 35 ppt is therefore about 25-26 kg/m3 denser than fresh water at the same temperature. Salt also lowers the temperature of maximum density below 0 C, which is why the ocean does not behave like a lake in winter.
Why does ice float if it comes from water?
When water freezes, hydrogen bonds lock molecules into a hexagonal lattice with more empty space than liquid water. Ice at 0 C has a density of about 917 kg/m3, well below liquid water at 0 C (999.8 kg/m3). Since ice is less dense than the water beneath it, it floats. This unusual property is critical for aquatic ecosystems: it insulates lakes and rivers, preventing them from freezing solid in winter.
How do I convert kg/m3 to g/cm3 or lb/ft3?
Divide kg/m3 by 1000 to get g/cm3 (so 998 kg/m3 = 0.998 g/cm3). Multiply kg/m3 by 0.062428 to get lb/ft3 (998 kg/m3 = 62.31 lb/ft3). This calculator outputs all three unit formats simultaneously so you never need to do the conversion manually.
What is specific gravity and how does it relate to density?
Specific gravity (SG) is the ratio of a substance's density to the density of pure water at 4 C (the reference, 999.975 kg/m3). It is dimensionless. For water at 20 C with density 998.2 kg/m3, SG = 998.2 / 999.975 = 0.99823. An SG below 1.0 means the substance is less dense than the reference water; above 1.0 means denser. Hydrometers used in brewing and battery testing measure specific gravity directly.
Sources
- Kell, G.S. (1975) Density, thermal expansivity, and compressibility of liquid water from 0 to 150 C - J. Chem. Eng. Data 20(1)
- Millero, F.J. et al. (1980) The density of seawater at atmospheric pressure - Deep-Sea Research 27A
- IAPWS - International Association for the Properties of Water and Steam, Release on the IAPWS Formulation 1995