Water is densest at about 4 °C, which is why lakes freeze from the top and fish survive the winter below. Dissolved salt makes seawater about 2.5% denser, and pressure squeezes it slightly more in the deep ocean.
How to use the Water Density Calculator
- Temperature
- Salinity (g/kg)
- Gauge pressure
- Object density (optional)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the temperature, salinity (35 g/kg for typical seawater) and gauge pressure, and optionally the density of an object.
Inputs explained
| Field | What to enter |
|---|---|
| Temperature | Units: °C, °F, K |
| Salinity (g/kg) | A number |
| Gauge pressure | Units: bar, MPa, psi, kPa |
| Object density (optional) | Units: kg/m³, g/cm³, g/mL, lb/ft³, lb/in³ |
What each result means
- Water density = [rs]×(1 + 4.6e-10×[Gauge pressure])
- Specific gravity (vs. 4 °C fresh water) = [Water density]÷999.97
Example values
How the main result changes when Temperature changes and everything else stays at the example values above:
| Temperature (°C) | Water density (kg/m³) |
|---|---|
| 10 | 999.7027 |
| 15 | 999.1026 |
| 20 | 998.2067 |
| 25 | 997.047 |
| 30 | 995.6488 |
| 40 | 992.2152 |
Formula
Fresh water (Tanaka 2001): ρ = 999.975·[1 − (T − 3.983)²(T + 301.797)/(522528.9·(T + 69.349))]
Salinity (UNESCO EOS-80 terms) and compressibility ≈ 4.6 × 10⁻¹⁰ Pa⁻¹ are added
Worked example
Inputs — Temperature: 20 °C; Salinity (g/kg): 0; Gauge pressure: 0 bar; Object density (optional): 1,000 kg/m³.
Results — Water density = 998.206746 kg/m³; Specific gravity (vs. 4 °C fresh water) = 0.998237; The object = Sinks.
Key terms
- Salinity
- Mass of dissolved salts per kilogram of seawater.
Frequently asked questions
At what temperature is water densest?
About 3.98 °C (999.97 kg/m³).
How dense is seawater?
About 1,025 kg/m³ at the surface.
Does pressure change water density much?
Only slightly – about 0.46% per 100 bar.
Why does ice float?
Ice is about 9% less dense than liquid water.