Seawater Freezing Point Calculator
The temperature at which seawater of a given salinity starts to freeze, at the surface or at depth, from the UNESCO equation.
Results
Freezing point of the seawater
-1.922
°C
Freezing point in °F
28.54
°F
How far below 0 °C it freezes
1.922
°C
The dashed line marks the value you entered.
What this tool does
Salt lowers the freezing point: ocean water at a salinity of 35 freezes at about −1.92 °C, not 0 °C, and the more saline brine left behind as ice forms freezes even lower. Pressure lowers it further, by about 0.075 °C for every 100 dbar — roughly 100 m — which is why water can stay liquid below an ice shelf at a temperature that would freeze at the surface. The equation is the UNESCO 1983 standard and is meant for salinities between about 4 and 40.
Formula
T_f = −0.0575 S + 1.710523×10⁻³ S^1.5 − 2.154996×10⁻⁴ S² − 7.53×10⁻⁴ p (S practical salinity, p in dbar; UNESCO 1983)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
sa | Salinity (open-ocean water is about 35) | PSU |
pb | Pressure (in decibar, about the same number as the depth in metres) | dbar |
TC | Freezing point of the seawater | °C |
TF | Freezing point in °F | °F |
DR | How far below 0 °C it freezes | °C |
Worked example
- Salinity (open-ocean water is about 35)35 PSU
- Pressure (in decibar, about the same number as the depth in metres)0 dbar
- Freezing point of the seawater-1.922 °C
- Freezing point in °F28.54 °F
- How far below 0 °C it freezes1.922 °C
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.