Seawater Density from Temperature & Salinity Calculator

Formula used

Density is mass divided by volume. The calculator’s primary TEOS-10 path evaluates the computationally efficient 75-term polynomial for specific volume from Absolute Salinity, Conservative Temperature, and sea pressure. Density is then the reciprocal of specific volume.

For legacy work, the EOS-80 path first calculates density at atmospheric pressure from Practical Salinity and in-situ temperature. It then applies the secant bulk modulus to account for compression at pressure. The simplified option uses a deliberately transparent linear estimate and is not presented as a standards-grade equation.

Variables

ρ

Seawater density, normally reported in kilograms per cubic metre.

v

Specific volume, measured in cubic metres per kilogram.

SA

Absolute Salinity in grams of dissolved material per kilogram of seawater.

SP

Practical Salinity on PSS-78. It is dimensionless even when informally described as PSU.

CT

Conservative Temperature on the ITS-90 Celsius scale.

p

Sea pressure in decibars, excluding one standard atmosphere.

σ

Density anomaly, commonly density minus 1000 kg/m³.

How pressure and depth are handled

A decibar is close to the pressure change caused by one metre of seawater, but the equivalence is not exact. This page estimates pressure from depth with latitude-dependent gravity and a representative seawater density. For instrument-quality work, enter measured sea pressure directly. Atmospheric pressure units are converted to their decibar equivalent, while the calculation itself treats the entered value as sea pressure unless the selected unit explicitly represents depth.

How to use this calculator

1. Choose the appropriate workflow

Quick mode is designed for a temperature, salinity, and optional surface-pressure estimate. Advanced mode exposes salinity representation, temperature type, location, reference pressure, uncertainty, and mass-volume tools. Compare mode adds a second water sample. Reverse mode searches numerically for salinity, temperature, or pressure that reaches a target density. Batch mode processes pasted rows.

2. Select a scientific standard

Use TEOS-10 when you can provide or reasonably derive Absolute Salinity and Conservative Temperature. Use EOS-80 when reproducing historical datasets that explicitly used Practical Salinity and in-situ temperature. The simplified equation is suitable for demonstrations, rough checks, and interface testing. Freshwater mode intentionally removes the salinity contribution.

3. Enter temperature and salinity carefully

Confirm both the value and its unit. A salinity written as 3.5 percent corresponds to about 35 grams per kilogram, not 3.5 PSU. Practical Salinity is not a mass fraction. When TEOS-10 is selected and Practical Salinity is entered, this page converts it to Reference Salinity. For the Baltic region, the official analytic conversion is available. Elsewhere, enter a known Absolute Salinity anomaly when one is available.

4. Add pressure, depth, or coordinates

Leave pressure at zero for a sea-surface calculation. Enter decibars for a measured subsurface state, or choose a depth unit for an estimate. Latitude affects the depth conversion. Longitude and latitude also determine whether the Baltic analytic salinity conversion can be applied.

5. Review cautions, assumptions, and output units

Read every warning below the result. The page identifies legacy calculations, simplified calculations, unusual values, and states outside the recommended TEOS-10 polynomial funnel. Change decimal precision only for presentation; extra displayed digits do not create extra physical accuracy. Use uncertainty fields to estimate how sensor errors propagate into density.

6. Save or compare results

Copy the summary, download JSON or CSV, print the report, or create a shareable URL. Browser history stores only compact recent results on the local device. Batch results can be exported separately. The chart can reveal how density responds to temperature, salinity, pressure, or estimated depth while other inputs remain fixed.

Understanding seawater density

Seawater density describes how much mass occupies a given volume. Ocean water is usually denser than pure freshwater because dissolved salts add mass and alter molecular packing. Temperature, salinity, and pressure interact rather than acting as completely independent corrections. For that reason, professional oceanography uses an equation of state instead of adding three unrelated adjustment factors.

Temperature normally has an inverse relationship with seawater density. Heating increases molecular motion and tends to expand the water, so the same mass occupies a larger volume. Cooling usually makes seawater denser until freezing processes become important. Unlike freshwater near four degrees Celsius, ordinary ocean salinity changes the temperature of maximum density and pushes the freezing point below zero Celsius.

Salinity usually increases density. A warm, salty water mass can therefore have a density similar to colder, fresher water. This compensation is one reason temperature-salinity diagrams are valuable. Contours of constant density reveal combinations that may remain neutrally buoyant relative to each other even though their temperatures and salinities differ substantially.

Pressure compresses seawater. The effect is small near the surface but accumulates through thousands of metres of depth. In-situ density at depth includes that compression. Potential density removes the direct compression effect by evaluating a parcel at a chosen reference pressure while retaining its thermodynamic water-mass properties. Oceanographers use sigma notation to report density anomalies more compactly.

Practical Salinity and Absolute Salinity are not interchangeable labels. Practical Salinity is derived from conductivity ratios and is dimensionless. Absolute Salinity represents mass fraction in grams per kilogram and accounts for regional composition differences. Reference Salinity is a fixed scaling of Practical Salinity. In much of the open ocean, Reference Salinity is a close first estimate of Absolute Salinity, but regional anomalies matter when high accuracy is required.

Conservative Temperature is designed to represent heat content more consistently than potential temperature. Exact conversion among in-situ temperature, potential temperature, and Conservative Temperature requires the full Gibbs SeaWater routines. This standalone page accepts Conservative Temperature directly for the cleanest TEOS-10 density path. When another temperature type is selected, it reports that a compact conversion estimate was used rather than hiding the approximation.

Density results should always be interpreted with measurement quality in mind. Conductivity, temperature, and pressure sensors have calibration limits, response times, and drift. Sample handling can introduce evaporation or temperature changes. The uncertainty calculator uses local numerical derivatives and root-sum-square propagation, which is useful for independent small errors but does not replace a complete metrology analysis.

In engineering, density affects pump loads, buoyancy, tank mass, desalination processes, sonar propagation, and hydrometer readings. In ocean science, density differences influence stratification, mixing, circulation, and the vertical stability of water masses. A denser parcel tends to sink below a lighter parcel, but real flows also depend on turbulence, currents, boundaries, and double-diffusive effects.

The calculator therefore provides both one-number convenience and context. The main density result is paired with specific volume, potential density, sigma values, thermal expansion, saline contraction, compressibility, estimated sound speed, freezing temperature, viscosity, and freshwater buoyancy difference. Some secondary properties use compact empirical estimates and are explicitly marked as estimates. The core TEOS-10 density polynomial and EOS-80 density calculation remain separate, selectable methods.

Frequently asked questions