Seawater Freezing Point Calculator
Find the freezing point of seawater from salinity and pressure, check how far your water is from freezing, or get the brine salinity and volume inside sea ice.
🧊 What is the Freezing Point of Seawater?
The freezing point of seawater is the temperature at which liquid seawater first begins to form ice crystals. Unlike fresh water, which freezes at 0 °C, seawater freezes at a lower temperature because dissolved salts reduce the activity of the water molecules, a colligative effect called freezing point depression. Normal open ocean water with a practical salinity of 35 psu freezes at about −1.92 °C at the surface. Because the ice that forms is nearly pure water, the salt is rejected into the remaining liquid, which makes it saltier and keeps it liquid to even lower temperatures.
Oceanographers, polar scientists and engineers use this number constantly. Ocean and climate models need it to decide when sea ice forms and how much latent heat is released. Ships and offshore platforms use it to judge icing risk and the freezing of ballast and sea chests. Researchers working under Antarctic ice shelves use the pressure term to understand why water at −2.3 °C can sit against ice hundreds of metres thick without freezing. Aquaculture and desalination designers use it for brine and chilled seawater systems, and sea ice scientists use the inverse relation to find the salinity of the brine trapped inside ice.
Two points are often misunderstood. First, the freezing point is not constant for the ocean: it moves by about 0.057 °C for each practical salinity unit near 35, and by 0.075 °C for each 100 m of depth. Second, reaching the freezing point does not guarantee ice forms. Clean seawater can supercool by a fraction of a degree until ice nuclei appear, and in turbulent polar water this produces frazil ice crystals suspended through the mixed layer. The supercooling margin in this calculator, your water temperature minus the freezing point, tells you how close water is to that threshold.
This calculator has two modes. The freezing point mode applies the UNESCO formula to a salinity and pressure and reports the salt and pressure contributions and your supercooling margin. The sea ice brine mode inverts the same formula numerically to find the salinity of the brine that coexists with ice at a given temperature, then estimates the brine volume fraction of the ice. For density, which also shifts with salinity and temperature, see the seawater density calculator (EOS-80), which reports a freezing point as a side output.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Open Ocean Surface Water
S = 35 psu at the surface (0 dbar), water temperature 2 °C
Example 2 — Arctic Fjord Surface Layer
S = 30 psu at 0 dbar, water temperature −1.9 °C
Example 3 — Brackish Baltic Sea
S = 7 psu at 0 dbar, water temperature 0.5 °C
Example 4 — Under an Ice Shelf at 500 dbar
S = 34.5 psu at 500 dbar, water temperature −2.2 °C
Example 5 — Brine in First Year Sea Ice at −5 °C
Ice temperature −5 °C, bulk ice salinity 6 ppt
❓ Frequently Asked Questions
🔗 Related Calculators
What is the freezing point of seawater?
Seawater with a salinity of 35 psu at the surface freezes at about -1.92 degrees C. Lower salinity freezes warmer: brackish water at 7 psu freezes near -0.38 degrees C. Higher pressure lowers the freezing point by 0.000753 degrees C per dbar.
Why does salt water freeze at a lower temperature than fresh water?
Dissolved salt lowers the chemical activity of water, so liquid water stays in equilibrium with ice only at a colder temperature. This is freezing point depression, and it depends on how many dissolved particles there are, not on which salt they are.
What formula is used for the freezing point of seawater?
The UNESCO formula of Fofonoff and Millard (1983): Tf = -0.0575 S + 1.710523e-3 S^1.5 - 2.154996e-4 S^2 - 7.53e-4 p, with S in psu and p in dbar. At S = 40 and p = 500 dbar it gives -2.58857 degrees C.
How does pressure change the freezing point of seawater?
Each dbar of pressure lowers the freezing point by 7.53e-4 degrees C, because ice is less dense than water. At 500 dbar (about 500 m depth) that is 0.3765 degrees C, which is why water under thick ice shelves can be colder than -2 degrees C and still liquid.
What is supercooling in seawater?
Supercooling is liquid water colder than its freezing point. The supercooling margin here is the water temperature minus the freezing point. Positive means safely liquid, negative means below freezing, so ice should form or the water is supercooled.
At what salinity is the freezing point equal to the temperature of maximum density?
The two curves cross near 24.7 psu. Below that salinity, water is densest above its freezing point like fresh water. Above it, as in normal seawater, the density increases right down to the freezing point, so cooling keeps driving convection.
What is the salinity of the brine inside sea ice?
It is fixed by temperature, because brine is in equilibrium with ice. At -5 degrees C the brine is about 83.5 psu, at -10 degrees C about 146 psu. The colder the ice, the saltier and the smaller in volume the brine pockets become.
How is brine volume of sea ice calculated?
Frankenstein and Garner (1967) give vb = Si (49.185 / |T| + 0.532), with Si the bulk ice salinity in ppt, T the ice temperature in degrees C and vb in parts per thousand. It is valid from -22.9 to -0.5 degrees C.
What is the 'law of fives' for sea ice?
Sea ice becomes permeable to fluid flow when the brine volume exceeds about 5 percent, which happens near -5 degrees C at a bulk salinity of 5 ppt. Below that threshold the brine pockets are mostly disconnected.
Does the freezing point of seawater depend on the ocean or on the salt mix?
Only slightly. Open ocean seawater has a nearly constant salt ratio, so salinity alone sets the freezing point to within a few thousandths of a degree. Lakes and estuaries with different ion ratios can deviate a little from this formula.
Is the UNESCO formula valid for brackish water such as the Baltic Sea?
The fit was developed for salinities of 4 to 40 psu, so the Baltic at 7 psu is inside the range. Below 4 psu the formula still behaves sensibly and reaches 0 degrees C at zero salinity, but it is an extrapolation.