# What is the average salinity of the ocean?

> The average salinity of the open ocean is about 35, meaning roughly 35 grams of dissolved salts are present in one kilogram of seawater. This is often described as 35 parts per thousand or about 3.5 percent by mass. The value is...

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Published: 2026-08-27T13:51:56+00:00
Categories: Explainer, Oceans

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The average salinity of the open ocean is about 35, meaning roughly 35 grams of dissolved salts are present in one kilogram of seawater. This is often described as 35 parts per thousand or about 3.5 percent by mass. The value is a global reference, not a concentration found at every coast or depth.

NOAA's [salinity lesson](https://oceanservice.noaa.gov/education/dyw-pass-salt.html) uses the same average and emphasizes that sea salts include many dissolved ions. Sodium and chloride dominate, but magnesium, sulfate and smaller components contribute to the total.

Surface salinity commonly ranges from roughly 32 to 37 across the open ocean. River mouths and polar melt zones can be fresher, while enclosed hot seas may be saltier. Measurements also change with depth as water masses formed in different climates spread through the ocean.

## What a salinity value means

Salinity once meant the mass of dissolved material left after evaporating a sample. Modern oceanography commonly uses the **Practical Salinity Scale**, which derives a value from electrical conductivity relative to a standard solution. Practical salinity is reported without a formal unit.

For general readers, 35 parts per thousand remains a useful approximation. It equals 3.5 percent, not 35 percent. A liter of typical seawater weighs slightly more than a kilogram, so simple gram-per-liter comparisons are close but not exact.

Scientists may also use absolute salinity, expressed as grams per kilogram and calculated with information about water composition. The TEOS-10 standard uses absolute salinity with conservative temperature to calculate density and other thermodynamic properties more accurately.

## Why the global average is not a map value

An average combines large regions with different freshwater balances. Evaporation removes water and raises salinity, whereas precipitation lowers it. River discharge produces fresh plumes that can extend far beyond a delta.

Sea ice rejects much of its salt as it freezes, increasing salinity beneath the forming ice. Melting adds fresh water during warmer seasons. Ocean currents then transport these signals away from their source.

The World Ocean Atlas compiles quality-controlled profiles into climatological fields. NOAA's [atlas description](https://www.ncei.noaa.gov/products/world-ocean-atlas) explains that its salinity maps are analyzed means rather than a snapshot of one day. A map cell represents observations combined across a chosen period and depth.

Global averaging also depends on how areas and depths are weighted. A simple average of stations would overrepresent heavily sampled coasts. Researchers use gridded fields and volume information to make the estimate physically meaningful.

## Surface salinity follows the water cycle

Subtropical ocean regions often have the saltiest broad surface waters because evaporation exceeds rainfall. The Atlantic is generally saltier than the Pacific at comparable latitudes, reflecting differences in atmospheric moisture transport and basin circulation.

Near the equator, heavy rainfall creates a relative salinity minimum despite warm temperatures. High latitudes are freshened by precipitation and ice melt. The Baltic Sea and major river estuaries fall well below open-ocean values because their connection with saltwater is limited.

The Red Sea and Persian Gulf reach high salinities under intense evaporation with restricted exchange. These regional extremes do not overturn the global reference; they demonstrate why a single number cannot describe local conditions.

## Salinity changes below the surface

A layer with a rapid vertical salinity change is called a **halocline**. In warm subtropical oceans, salty surface water may lie above fresher intermediate water. At high latitudes, fresh surface water can overlie saltier deep water.

Temperature and salinity together set density. A stable ocean generally has lighter water above denser water, even if the salinity pattern alone seems reversed. Vertical mixing can weaken gradients, while heating or freshwater input rebuilds them.

Deep water carries the signature of its formation region. Dense North Atlantic waters are relatively salty, whereas water formed around Antarctica has a different temperature-salinity relationship. Oceanographers use these properties to trace circulation long after the water leaves the surface.

## How salinity is measured

A shipboard **CTD** records conductivity, temperature and pressure as it descends. Water bottles mounted around the frame collect samples used to calibrate sensors. These profiles provide high vertical resolution at one location.

Argo floats repeat profiles through the upper 2,000 meters across most ice-free oceans. Moorings sample rapidly at fixed points, while autonomous gliders cover regional transects. Together, the systems capture changes that a ship survey could miss.

Satellites infer sea-surface salinity from microwave emissions. NASA's [Aquarius mission](https://science.nasa.gov/mission/aquarius) could detect changes near 0.2 practical salinity units over broad areas. Satellite estimates need corrections for surface temperature, roughness and other effects, then comparison with in-water data.

## Why a few tenths can be important

Small salinity differences alter seawater density. Density gradients influence vertical mixing and the formation of deep currents. A surface anomaly can also reveal where rain has fallen or where a river plume is moving.

Marine organisms regulate internal water and ion concentrations. Some tolerate a wide range, while others experience stress after relatively small changes. Salinity monitoring is especially important in estuaries and coastal farms where conditions shift quickly.

The value 35 is best treated as the ocean's baseline. It gives scale to a sample, but location and depth provide the meaning. A reading of 33 may be ordinary near a rainy coast and a strong freshwater signal in a dry subtropical basin.

## Common salinity comparisons can mislead

Comparing seawater with a spoonful of table salt captures scale but not composition. Ocean salts include many ions and table salt is mainly sodium chloride. A kitchen mixture also lacks the pressure, gases and biological processes present in the sea.

Percent, parts per thousand and practical salinity should not be mixed without labels. A value of 3.5 percent corresponds to about 35 parts per thousand. Writing 35 percent exaggerates normal seawater concentration by a factor of ten.

Temperature does not directly change the amount of salt in a sealed sample, yet evaporation associated with warm dry conditions can raise salinity. Temperature also changes density and conductivity, so instruments correct for it before reporting a value.

Averages can conceal freshwater layers that influence weather or ecosystems. Researchers retain full profiles and local time series rather than replacing every observation with 35.

The useful question is often how far a measurement departs from the expected local value. A change of **0.2 salinity units** may be meaningful even though both measurements round to 35 in a general description.

## How to interpret a reported average

A paper should specify whether its number refers to surface water, a depth range or the entire ocean volume. The phrase global mean can describe different calculations. **Area-weighted surface salinity** is not identical to a volume-weighted value.

Sampling period also matters. An annual average smooths monsoon rain and seasonal ice melt. A monthly mean retains those cycles, while a daily field may reveal fronts that disappear under longer averaging.

Uncertainty belongs with the final digits. Writing 35 communicates an approximate global baseline. Extra decimal places imply precision that a broad description may not support, especially when products use different **salinity definitions**.

## Salinity is linked to density

Dissolved salts add mass and alter water properties. At the same temperature and pressure, saltier seawater is generally denser. Density differences help maintain **ocean stratification** and influence whether surface water sinks.

Temperature often has the larger local effect, so salinity should be interpreted with both temperature and pressure. Oceanographers calculate **potential density** to compare water parcels after accounting for compression at depth.

Density measurements also expose instrument errors. If temperature and salinity imply an impossible vertical instability across a broad calm layer, technicians inspect sensor calibration and sampling history before accepting the profile. This cross-check protects the long-term **ocean salinity record**.

**Related reading:** [how saltwater lakes form](https://www.argo.net/what-are-saltwater-lakes-and-how-do-they-form/) and [the saltiest ocean in the world](https://www.argo.net/the-saltiest-ocean-in-the-world/).

 **Explore this topic:** [How do satellites measure ocean salinity?](https://www.argo.net/how-do-satellites-measure-ocean-salinity/) and [Why are rivers not salty?](https://www.argo.net/why-are-rivers-not-salty/).