Seawater Density Calculator

At typical ocean salinity (above roughly 27 PSU), yes: this calculator's Density output falls as Temperature rises across the entire -2°C to 40°C range at fixed salinity and pressure, and cold water tends to sink beneath warmer water, driving large-scale thermohaline circulation. Below that salinity the picture changes -- fresh and brackish water have a genuine density maximum near 4°C (this is why lakes freeze from the top down: 4°C water sinks below both colder and warmer water). Try Salinity at 0 with Temperature stepped from -2°C upward and Density rises to a peak around 4°C before it starts falling, exactly reproducing that freshwater behavior. As Salinity increases, the density-maximum temperature drops and moves below this calculator's -2°C floor once Salinity passes about 26-27 PSU, which is why the simple "colder is always denser" rule only holds at near-oceanic-and-saltier salinities.

Sigma-t is simply Density minus 1000 kg/m³, so a Sigma-t of 26 means the water is denser (about 1026 kg/m³) than water with a Sigma-t of 20 (about 1020 kg/m³). Oceanographers use Sigma-t instead of raw density because seawater density values all cluster near 1000-1030 kg/m³, and subtracting the constant makes the meaningful differences between water masses easier to read and compare at a glance.

Decibars are the oceanographic convention precisely because 1 decibar of hydrostatic pressure in seawater works out to very close to 1 meter of depth, so entering Pressure in decibars lets you treat the number as an approximate depth in meters without a separate conversion step. This calculator's Pressure input covers 0 to 11,000 dbar, roughly surface to the deepest ocean trenches.

Yes, across this calculator's full 0 to 42 PSU range at any fixed temperature and pressure: dissolved salt adds mass without adding much volume, so Density rises monotonically as Salinity increases. This is why the very salty Red Sea (around 40 PSU) produces denser water than the fresher Baltic Sea, even at similar temperatures.

No -- it is a simplified version. The pure-water and salinity polynomial terms follow the real UNESCO/EOS-80 structure -- the equation of state the Joint Panel on Oceanographic Tables and Standards published in 1980 and the Intergovernmental Oceanographic Commission used as the official standard until TEOS-10 replaced it in 2009 -- but the pressure correction here is a linear approximation rather than the full pressure-dependent bulk modulus terms in the complete standard, so results near the surface are close to reference values while deep, high-pressure results should be treated as an educational approximation rather than a lab-grade calculation.