What equation of state is used?

The calculator uses the simplified UNESCO/EOS-80 polynomial equation of state for seawater, which models density as a function of temperature, salinity (PSU), and pressure (dbar). It is accurate to ±0.005 kg/m³ for typical ocean conditions.

What is sigma-t (σt)?

Sigma-t is defined as (density − 1000) kg/m³. It simplifies reporting because open-ocean surface water is typically 1024–1028 kg/m³, so sigma-t values like 24–28 are easier to read than the full density.

What is a typical seawater density?

Open ocean surface seawater is typically 1025–1028 kg/m³ (salinity ~35 PSU, temperature 10–25 °C). Fresh water at 4 °C is approximately 1000 kg/m³. Polar brine can exceed 1030 kg/m³.

How does pressure affect density?

Pressure compresses seawater slightly. At 1000 dbar (≈ 1000 m depth) the pressure effect adds roughly 4–5 kg/m³ compared to the surface value at the same temperature and salinity.

What are valid input ranges?

The equation is reliable for temperature −2 to 40 °C, salinity 0 to 42 PSU, and pressure 0 to 10,000 dbar (≈ 10,000 m). Inputs outside these ranges produce a warning.

Seawater Density Calculator

Name: Seawater Density Calculator
Availability: InStock
Author: Nham Vu

Enter temperature, salinity, and pressure to calculate seawater density using the standard oceanographic equation of state.

Input Parameters

Temperature (°C)

Valid range: −2 to 40 °C

Salinity (PSU)

Practical salinity units: 0 = fresh, ~35 = open ocean, up to 42

Pressure (dbar)

1 dbar ≈ 1 m depth; use 0 for sea surface

Results

Enter values and click Calculate

Quick Reference: Typical Seawater Densities

Water Type	Temp (°C)	Salinity (PSU)	Density (kg/m³)
Fresh water (max density)	4	0	999.97
Tropical ocean surface	28	35	1022.9
Temperate ocean surface	15	35	1025.9
North Atlantic deep water	2	35	1027.8
Polar brine	-1.5	40	1032.3

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Summary

Enter temperature, salinity, and pressure to calculate seawater density using the standard oceanographic equation of state.

How it works

Enter water temperature in degrees Celsius (0–40 °C).
Enter salinity in practical salinity units (PSU), typically 0–42 for seawater.
Enter pressure in decibars (1 dbar ≈ 1 m depth); use 0 for surface.
Click Calculate to compute density using the simplified equation of state.
Results show density in kg/m³ along with sigma-t and buoyancy notes.

Use cases

Estimate seawater density at a given ocean depth for diving or submersible planning.
Verify CTD sensor readings against expected density values.
Study how temperature and salinity affect stratification in estuaries.
Compute density differences for buoyancy and flotation calculations.
Support oceanography coursework and lab assignments.
Compare freshwater vs. brackish vs. oceanic salinity regimes.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu