Sea Salinity Explained: Definition, Average, and Key Facts

Rivers carry dissolved minerals to the ocean, yet the sea remains salty because water evaporates but salt stays behind – a process that has shaped Earth’s oceans for billions of years. The average salt concentration in the open ocean is about 35 grams per liter, according to the Science Learning Hub (New Zealand science education resource).

Average ocean salinity: 35 g/L (3.5%) ·
Highest open ocean salinity: Red Sea (~40 g/L) ·
Dead Sea salinity: ~340 g/L (salt lake)

The table below summarizes the essential salinity data.

What is the salinity of the sea?

Salinity measures the amount of dissolved salts in seawater. The most common unit is practical salinity units (PSU) or parts per thousand (ppt). The global average is 35 ppt, meaning 35 grams of salt per kilogram of seawater, as documented by the Science Learning Hub (New Zealand science education resource).

Understanding salinity units

• PSU (Practical Salinity Unit) is dimensionless and roughly equivalent to ppt.
• ppt (parts per thousand) is the older standard: 35 ppt = 3.5%.
• For most seawater, 35 PSU equals 35 grams of salt per liter.

Most of the ocean falls between 34 and 36 ppt, but regional differences are driven by evaporation, rainfall, and river inflow (Science Learning Hub).

Global average salinity

The Atlantic Ocean averages around 37 ppt at the surface in subtropical zones, making it the saltiest major ocean basin, according to the Woods Hole Oceanographic Institution (WHOI, a leading ocean research center). In contrast, the Arctic Ocean has the lowest salinity, sometimes below 30 ppt, due to melting ice and river runoff.

The pattern: latitude and circulation control surface salinity. High evaporation in the subtropics leaves salts behind, while heavy rainfall near the equator dilutes them.

Why is the ocean salty?

Rain and rivers dissolve minerals from rocks and carry them to the sea. This process has been running for billions of years, and the salt accumulates because water evaporates but salts stay behind. The U.S. Geological Survey (USGS, federal earth science agency) explains that rivers deliver about 4 billion tons of dissolved salts annually.

Weathering and river runoff

• Chemical weathering of continental rocks releases sodium, chloride, and other ions.
• Rainwater, slightly acidic, accelerates the breakdown.
• Rivers carry these ions to estuaries and eventually to the ocean.

The main dissolved salt is sodium chloride — table salt. Data from Connected Ocean (online oceanography resource) shows that chloride and sodium together make up about 85% of all dissolved solids in seawater.

Hydrothermal vents and volcanic activity

Underwater volcanoes and hydrothermal vents also add minerals. When seawater seeps into the oceanic crust, it heats up, reacts with rocks, and emerges with a new load of dissolved elements. NASA’s Scientific Visualization Studio notes that this deep-sea cycle complements the riverine input, though rivers are the dominant source.

How much salt is in 1 litre of seawater?

One liter of typical seawater contains about 35 grams of salt — roughly the weight of two tablespoons. That’s the global average from the Science Learning Hub.

Converting salinity to grams

• Salinity of 35 ppt = 35 grams of salt per kilogram of seawater.
• Since seawater density is about 1.025 kg/L, 1 liter holds slightly more than 35 grams — roughly 35.9 grams.

To put it in perspective: if you evaporated 10 liters of seawater, you’d get about 350 grams of salt — enough to fill a salt shaker many times over.

What does 40% salinity mean?

A 40% salinity figure — sometimes misread as 40 ppt — actually means 40 grams per 100 grams of water, which is 400 g/L. That is not found in open oceans. Such extreme levels occur only in hypersaline lakes like the Dead Sea. The WorldAtlas (geographic reference publisher) ranks the Dead Sea at about 340 g/L, which is 34% — still far below 40%.

Understanding the difference between ppt and percentage prevents common misunderstandings.

Which ocean has the highest salinity?

The Atlantic Ocean, particularly its subtropical belts in both hemispheres, has the highest average surface salinity among the five major oceans. Data from Woods Hole Oceanographic Institution shows Atlantic surface salinity can reach 37 ppt, while the North Pacific averages only 33 ppt.

Comparing ocean basins

The following table compares average surface salinity across the world’s major oceans.

The data reveals a clear pattern: as you move away from the equator, salinity rises in subtropical zones (20°–30° latitude) before dropping again in polar regions. The Science Learning Hub attributes this to strong subtropical winds that boost evaporation and scarce rainfall.

The Red Sea and Mediterranean Sea

These semi-enclosed seas have even higher salinity because of limited water exchange and extreme evaporation. The Red Sea reaches about 40 g/L, and the Mediterranean averages 38 g/L, according to WorldAtlas. But neither matches the extreme of the Dead Sea, which is technically a salt lake — not a sea — with salinity of roughly 340 g/L.

Can humans drink seawater?

No. The human body cannot process the high salt concentration without devastating consequences. The NOAA National Ocean Service (U.S. federal ocean agency) states plainly: “The human body cannot tolerate high concentrations of salt.”

Health risks of drinking seawater

• Kidneys require fresh water to flush out excess salt; seawater forces them to work harder and can lead to dehydration.
• Symptoms include diarrhea, vomiting, confusion, and kidney damage.
• In extreme cases, drinking seawater can be fatal.

The salt content of 35 g/L is about seven times the concentration of human blood. Your cells would lose water trying to balance the osmotic pressure, essentially leading to cellular dehydration.

Safe water alternatives at sea

Desalination plants convert seawater into drinking water on ships and in coastal cities, but without treatment, seawater is unsafe. Life rafts and survival kits often include manual desalinators or specify that seawater should never be consumed.

For anyone at sea, desalination is the only safe option to obtain drinking water.

What two seas cannot mix?

This question stems from a viral video showing a sharp color boundary between two bodies of water. In reality, the Atlantic and Pacific Oceans do mix — but slowly. The visible line is a halocline: a vertical zone where salinity changes abruptly, creating a density barrier.

The myth of non-mixing oceans

Popular descriptions of “two seas that never mix” often reference a verse in the Quran. Scientifically, water bodies with different salinity and temperature can maintain a temporary boundary due to density stratification. The Connected Ocean resource explains that in places like the Mediterranean outflow into the Atlantic, the saltier Mediterranean water sinks and spreads out, creating a stable layer that is distinct for hundreds of miles.

Scientific explanation: halocline and density gradients

• Salinity + temperature determine density. Colder, saltier water is denser.
• When two water masses meet with different densities, they form a pycnocline (density gradient).
• Sharp salinity contrasts create haloclines that can look like a “wall” between waters.

Over time, mixing does occur through turbulence and diffusion. The boundary is real but temporary — not a permanent separation.

For a broader look at how myths can persist, see our fact-check of Independence Day (United States): Myths vs History.

“The human body cannot tolerate high concentrations of salt.”

— NOAA National Ocean Service

“Rivers carry dissolved salts to the ocean. Over billions of years, the salt has accumulated.”

— U.S. Geological Survey

For any sailor, desalination plant operator, or coastal resident, the message is the same: seawater is a resource only after treatment. Its salt load is far beyond what our bodies or most crops can handle. Understanding sea salinity matters — coastal communities and desalination operators must grasp it to secure freshwater supplies in a warming world.