What Is the Difference Between a Sea and an Ocean?
In short: an ocean is a vast body of salt water bordered by continents, while a sea is smaller and usually partly enclosed by land. The clearest differences are size, borders and depth. Salinity varies from sea to sea, so it is not a reliable test.
The essentials
| Criterion | Sea | Ocean |
|---|---|---|
| Size | Smaller: the largest, the Philippine Sea, covers about 5.7 million km² | Immense: even the smallest, the Arctic, covers more than 15 million km² |
| Borders | Land and/or ocean, usually partly enclosed | Open, bordered by continents |
| Depth | Generally shallower (the English Channel reaches 174 m, the Mediterranean 5,267 m) | Often deeper than 7,000 m, down to 10,935 m in the Pacific |
| Salinity | Varies widely, from about 7 g/L (Baltic) to about 40 g/L (Red Sea) | About 35 g/L on average |
| Examples | Mediterranean, North Sea, Caribbean, Baltic | Atlantic, Pacific, Indian, Southern, Arctic |
You may be asking yourself this question right now, toes in the sand, with nothing but water on the horizon: is that the sea or the ocean? People use both words as if they meant the same thing, but geographers do not. Here is what really separates a sea from an ocean, with the numbers to back it up.
Sea vs ocean: different sizes and borders
The sheer size of an ocean is the easiest way to tell it apart from a sea.
Before the first voyage around the world, the ocean was known as the "ocean sea". It was while crossing the world ocean that Magellan named it the "Pacific", because the water he sailed on was so calm. Each part of the world ocean was later given its own name: Atlantic, Indian, Southern and Arctic.
To give you an idea of the scale, the largest sea, the Philippine Sea, covers about 5.7 million km², while the smallest ocean, the Arctic, covers more than 15 million km². An ocean is always bordered by continents. A sea is smaller and is usually found where the ocean meets the land, at the margins of the ocean. There are several kinds of sea:
- Marginal sea: an extension of the ocean along a coast (the English Channel, the North Sea, the Caribbean Sea).
- Inland sea: connected only to another sea or to the ocean through a narrow channel (the Baltic, the Black Sea, the Adriatic).
- Semi-enclosed sea: linked to the ocean by a narrow passage (the Mediterranean, through the Strait of Gibraltar).
- Enclosed sea: not connected to any sea or ocean (the Caspian Sea, the Aral Sea).
The Sargasso Sea is the one exception. It has no shores at all: it is an area of the North Atlantic defined by ocean currents, which is why the line between sea and ocean can sometimes be blurred.
Is the sea saltier than the ocean?
Ocean water averages about 35 g of salt per litre, but salinity varies a lot from one sea to another.
The salinity of seawater averages about 35 g/L in the open ocean, and ocean currents help to keep it fairly stable (this is called thermohaline circulation). To understand this better, read our article on ocean currents, or find out why ocean water is salty.
Some seas are indeed saltier. In hot, dry regions, strong evaporation removes water but leaves the salt behind, which concentrates it. That is why the Mediterranean reaches about 39 g/L and the Red Sea about 40 g/L.
But the opposite also happens. Where rivers and rainfall add a lot of fresh water, a sea can be far less salty than the ocean. The Baltic Sea is only about 7 g/L. So salinity is not a reliable way to tell a sea from an ocean.
Depth: where the difference shows
Oceans are generally much deeper than seas, although some seas are very deep too.
Seas vary a lot. The English Channel is only 174 m at its deepest and the North Sea about 700 m. The Black Sea reaches 2,212 m and the Mediterranean 5,267 m.
Oceans plunge much further. The deepest point of the Indian Ocean, the Java Trench, is about 7,190 m. In the Atlantic, the Puerto Rico Trench reaches about 8,378 m. The deepest point known on Earth, the Challenger Deep in the Mariana Trench (Pacific Ocean), is about 10,935 m down.
So even if humans were the ones to draw the line between seas and oceans, the differences are real. Which do you prefer, the sea or the ocean?
Keep exploring: how waves are formed, bioluminescence in the ocean, the most beautiful salt marshes in France and riptides and how to swim safely.
Bring the ocean home
Frequently asked questions
What is the main difference between a sea and an ocean?
An ocean is an immense body of salt water bordered by continents. A sea is much smaller and is usually partly enclosed by land, at the margins of the ocean.
How many oceans are there on Earth?
Five: the Pacific, Atlantic, Indian, Southern and Arctic oceans.
Is the sea saltier than the ocean?
Not always. The open ocean averages about 35 g/L. Seas in hot, dry regions such as the Red Sea (about 40 g/L) are saltier, while seas that receive a lot of fresh water, such as the Baltic (about 7 g/L), are much less salty.
Which is the largest sea in the world?
The Philippine Sea, in the western Pacific, covers about 5.7 million km². Even so, it is far smaller than the smallest ocean, the Arctic, which covers more than 15 million km².
What is the deepest place in the ocean?
The Challenger Deep, in the Mariana Trench in the Pacific Ocean, at about 10,935 m deep.
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