Seawater density calculator from salinity, temperature and pressure

It combines a standard polynomial approximation for seawater at the surface with a simple compressibility correction for pressure. The result is useful for quick oceanographic checks, buoyancy estimates, classroom work, and engineering screening where a reproducible approximation is more appropriate than a full thermodynamic standard. Inputs are checked explicitly, including rejection of negative salinity and temperatures below the calculator's 0 °C freezing boundary.

Enter salinity, temperature, and pressure consistently

Provide practical salinity in PSU, temperature in degrees Celsius, and pressure in decibars. Typical open-ocean salinity is near 35 PSU, while fresher coastal or estuarine water can be much lower. A pressure of 0 dbar represents the surface reference used by the polynomial. Ocean pressure increases by roughly one decibar per metre of depth, so a value such as 1000 dbar is broadly associated with deep water near 1000 metres, although the exact pressure-depth relationship varies with latitude and local density. The calculator accepts finite numeric values only and applies documented bounds suited to its simplified model. Salinity cannot be negative, temperature cannot be below 0 °C, and pressure cannot be negative. These checks prevent mathematically possible but physically misleading output. Enter measured values without unit conversions hidden in the numbers: do not submit kelvin as temperature, pascals as pressure, or a mass fraction as salinity. The returned object repeats the normalized inputs alongside density, which makes automated results easier to audit and prevents uncertainty about which units were used.

Understand the simplified equation of state

The calculation begins with a fifth-order polynomial for pure-water density as a function of temperature. It then adds salinity terms proportional to salinity, salinity raised to the power of one and a half, and salinity squared. This structure follows the familiar EOS-80 atmospheric-pressure density approximation and captures the main effects that cooler water is generally denser and dissolved salts add mass. The calculator next applies a deliberately simplified pressure correction using a constant 2.2 GPa bulk modulus. In decibar units, that correction divides surface density by one minus pressure divided by 220000. Pressure therefore increases the reported density because seawater is compressed. The response includes both density_at_surface_kg_m3 and density_kg_m3, allowing you to see the pressure contribution separately. Values are rounded to six decimal places so repeated calls and browser calculations remain stable. This is a deterministic analytic estimate: it uses no network service, lookup table, random value, or clock. The same inputs consequently produce the same JSON result every time.

Interpret the result and know its limits

Use density_kg_m3 as the pressure-adjusted estimate for the supplied conditions. Use density_at_surface_kg_m3 when you need to compare samples on the same zero-pressure basis or isolate the effects of salinity and temperature. The model is well suited to rapid comparisons, preliminary buoyancy calculations, sanity checks on sensor data, teaching demonstrations, and reproducible software tests. It is not a replacement for TEOS-10, the Gibbs Seawater library, or a certified oceanographic workflow. A constant bulk modulus does not capture the detailed way compressibility changes with temperature, salinity, and pressure, and practical salinity is not identical to absolute salinity. The fixed 0 °C input boundary is also a validation rule for this calculator, not a prediction of the true freezing point of every seawater sample; saline water can remain liquid below 0 °C. For navigation, custody transfer, high-accuracy hydrography, or research conclusions sensitive to small density differences, use an appropriate thermodynamic standard and calibrated measurements. Treat this result as an explicitly simplified estimate whose assumptions are visible and repeatable.