Ocean Density Salinity Temperature Calculator

Seawater density controls where a water parcel comes to rest in the vertical, and because the ocean is stratified almost everywhere, small density differences drive the global overturning circulation. No simple physical law predicts it, so the International Equation of State of Seawater, EOS-80, expresses density as an empirical polynomial: a pure water term rho_w that depends on temperature alone, plus salinity terms in S, S to the three halves, and S squared whose coefficients themselves vary with temperature, followed by a pressure correction through the secant bulk modulus. Oceanographers usually report sigma-t, which is density minus 1000 kg per cubic meter, because the interesting variation is confined to the last few digits. Open ocean surface density spans roughly 1021 to 1029 kg per cubic meter, corresponding to sigma-t of 21 to 29. The dominant ranges are temperature from about minus 2 degrees C in polar water to 30 degrees C in the tropics and salinity from 33 to 37 PSU, with the Red Sea near 40 and Baltic surface water below 10. One behavior matters especially. Seawater has no density maximum above its freezing point once salinity exceeds about 24.7 PSU, unlike fresh water which peaks at 4 degrees C, so polar surface water keeps densifying as it cools until it freezes. EOS-80 is valid from minus 2 to 40 degrees C, 0 to 42 PSU, and 0 to 1000 bar, and it does not account for regional variation in dissolved composition.

Quantitative seawater density work began with the Challenger expedition of 1872 to 1876, whose chemist William Dittmar analyzed 77 water samples and demonstrated in 1884 that the relative proportions of the major dissolved ions are nearly constant worldwide. That constancy, now called the principle of constant proportions or Marcet's principle after Alexander Marcet's earlier 1819 observation, is what allows a single salinity number to stand in for full chemical composition. Martin Knudsen led an international commission that published Hydrographical Tables in 1901, defining chlorinity-based salinity and supplying the density tables that oceanographers used for the next seventy years. The modern formulation came from a coordinated international effort in the late 1970s. The Joint Panel on Oceanographic Tables and Standards, sponsored by UNESCO, SCOR, ICES, and IAPSO, adopted the Practical Salinity Scale in 1978 and then the International Equation of State of Seawater in 1980, drawing on high-precision laboratory measurements by Frank Millero at Miami and by Alvin Bradshaw and Karl Schleicher at Woods Hole. EOS-80 achieved an accuracy near 0.004 kg per cubic meter and became the global standard for three decades. It was formally superseded in 2010 when the Intergovernmental Oceanographic Commission endorsed TEOS-10, which rebuilds the whole system on a thermodynamic potential, though EOS-80 remains widely used for continuity with historical data archives.