Sea Salinity Explained: Highest & Lowest Seas, FAQs

The ocean is salty, but its saltiness varies dramatically from one sea to another — the Red Sea approaches 40 ppt while the Baltic barely registers 7 ppt. This guide explains what sea salinity means, which waters are the saltiest, and why the difference matters for everything from floating to ocean currents.

Average ocean surface salinity: 35 ppt ·
Maximum sea salinity: ~40 ppt (Red Sea, Persian Gulf) ·
Dead Sea salinity: ~340 ppt (≈10× average ocean) ·
Baltic Sea salinity: ~7 ppt (lowest of all seas)

Key facts at a glance

Four numbers capture the global salinity range, from nearly fresh to ten times saltier than the open ocean.

What do you mean by 40% salinity?

When someone says “40% salinity,” they almost certainly mean 40 parts per thousand (ppt) — which is 4% by mass, not 40%. Typical ocean water is around 35 ppt, or 3.5%. The Dead Sea, with about 340 ppt, is 34% salt — that’s the only natural water body that comes close to a 40% figure. A 40% salinity would mean 400 grams of salt per liter, which does not occur naturally in any sea on Earth (Wikipedia (encyclopedic reference)).

The implication: confusion between parts per thousand and percentage persists in casual language.

Which sea has the highest salinity in the world?

The Dead Sea takes the crown, but several other seas also rank far above the global average. Here’s how they stack up.

Dead Sea salinity

• The Dead Sea is the saltiest large water body on Earth, with salinity often cited at about 340 ppt — roughly 10 times saltier than average ocean water (National Geographic (travel & science publisher)). Popular descriptions put it at 34% by mass (National Geographic Education Blog (educational resource)).

What is 10 times saltier than seawater?

The Dead Sea is about 10 times saltier than average ocean water (340 ppt vs 35 ppt). This extreme concentration creates buoyancy that lets people float effortlessly.

Red Sea salinity

• The Red Sea is widely regarded as one of the saltiest seas, with surface salinity commonly ranging from 35 to 41 ppt. Deep water in the northern Red Sea can reach around 40.6‰ during winter cooling (ScienceDirect (academic research platform)). OceanX describes it as the hottest and saltiest sea on Earth (OceanX (ocean exploration nonprofit)).

Mediterranean Sea salinity

• The Mediterranean averages around 38 ppt, driven by high evaporation and limited freshwater inflow (Science Learning Hub (New Zealand science education site)).

Persian Gulf salinity

• The Persian Gulf also reaches about 40 ppt, making it comparable to the Red Sea (NOAA JetStream (US weather and ocean education)).

Baltic Sea salinity

• At the other extreme, the Baltic Sea is a brackish inland sea. Open surface waters typically range from 7 to 8 ppt in the Baltic Proper, dropping to 0–2 ppt in the northern Bothnian Bay (HELCOM State of the Baltic Sea (regional environmental authority)). Many rivers drain into it, diluting the salt (Britannica Kids (educational encyclopedia)).

Are salt and salinity the same?

Not exactly. “Salt” refers to a specific chemical compound — sodium chloride (NaCl) — which is the dominant salt in seawater. “Salinity” is the total concentration of all dissolved salts (including magnesium, calcium, and potassium) in water, expressed in parts per thousand or practical salinity units (NASA Salinity (space agency research)). Think of salt as one ingredient, and salinity as the whole recipe.

Which has more salinity, ocean or sea?

Enclosed seas can be much saltier — or much fresher — than the open ocean. The open ocean averages 35 ppt, with a typical range of 34 to 36 ppt (Science Learning Hub (New Zealand science education site)). But landlocked seas like the Dead Sea (340 ppt) and Red Sea (40 ppt) far exceed the ocean, while the Baltic Sea (7 ppt) is significantly lower. It depends on the balance between evaporation and freshwater inflow.

The catch: even small salinity differences can drive major oceanographic processes.

Can you swim in the saltiest sea?

Yes — and you’ll float without effort. The Dead Sea’s extreme salinity (340 ppt) makes water so dense that people bob like corks. But swimming requires caution: don’t splash water into your eyes or swallow it, and avoid staying in too long because the salt can irritate skin (Skyhook Adventure (travel safety guide)). It’s less a swim and more a hover.

Salinity across seas: a comparison

Five major water bodies, one spectrum: from nearly fresh to ten times saltier than the ocean.

The pattern: seas with high evaporation and limited freshwater input are saltiest; those with large river inflows are freshest. The Baltic’s extreme low salinity — below 2 ppt in some bays — is comparable to freshwater (Wikipedia (encyclopedic reference)).

Sea water salinity is expressed as a ratio of salt (in grams) to liter of water, written parts per thousand (ppt).

— NOAA JetStream (US weather and ocean education)

Salt dissolved in seawater – measured as salinity – drives currents that distribute heat and carbon around the globe.

— NASA Salinity (space agency research)

Most seawater has about 35 g of salt in every 1,000 g of water.

— Science Learning Hub (New Zealand science education site)

Sea salinity isn’t a single number — it’s a gradient that shapes ecosystems, currents, and even human activities like swimming and desalination. For anyone exploring marine environments, the key takeaway is that evaporation and freshwater inflow create extreme pockets of saltiness and freshness. The Dead Sea will always let you float effortlessly, the Baltic will feel almost fresh, and the open ocean sits reliably in the middle. Understanding those differences helps make sense of ocean circulation and the creatures that thrive — or struggle — in each zone. The Blue-Footed Booby Facts article shows how ocean salinity indirectly shapes marine bird habitats.

For readers converting salinity figures between units, our Gallon to Litre Conversion guide may be handy when dealing with volume-based measurements.