Sea vs Lake: What Are the 5 Differences?

© krisbiantoandy/

Written by Kyle Glatz

Updated: September 28, 2023

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Over 70% of the world is covered by water, with oceans comprising 96% of all surface water. We can break down the bodies of water by strict definitions, like lakes, ponds, and rivers. However, when we look at a sea vs lake, the definitions start to blur together. After all, the Caspian Sea is technically the world’s largest lake, even though it is often called a sea. If that’s the case, how can we tell them apart?

Seas are substantial bodies of water that is at least partially enclosed by a landmass. They are large bodies of water within an ocean. For example, the Oman Sea is a northern part of the Indian Ocean, but it is enclosed on three sides by Oman, Pakistan, Iran, and India.

We’re going to look at five major differences between a sea and a lake to show you how to distinguish one from the other.  

Comparing a Sea and a Lake

Seas are much deeper than lakes.
Size– Can be millions of square miles, such as the Philippine Sea, or as small as the Baltic Sea at 149,000 square miles
– Water is usually several hundred feet deep
– Typically, they are larger than a pond but smaller than a sea
– 20ft deep or more
– 200+ acres (0.5 square miles) is an accepted minimum size
– No limit on the size of the body of water
– The largest lake is 143,244 square miles
Water Type– Saltwater only– Freshwater (most often)
– Saltwater
– Brackish
Boundaries– Connected to the larger interconnected oceans
– Partially enclosed by landmasses  
– Completely enclosed by a landmass
– Water supplied by streams, rivers, and rain
– No opening
Formation– Natural only– Natural
– Manmade through damming
Current– Natural ocean current that is driven by winds, salinity, temperature, and more– Current primarily driven by wind

The Key Differences Between a Sea vs Lake

Lake Clark National Park in Fall

Lakes are bodies of water that are bounded by land.

©Jesse Hasup/

The greatest differences between a sea and a lake are their formation, boundaries, and size. Seas are vast naturally occurring bodies of saltwater that are partially enclosed by landmasses that are anywhere from 100,000 square feet and larger. Lakes are naturally occurring or manmade bodies or freshwater or saltwater that are completely enclosed by land and tend to be anywhere between 0.5 square miles and 143,244 square miles in size.

These differences are vast and noticeable. However, they are not the only ones applicable in this case. We’re going to dig deeper into these unique qualities and explore others.

Sea vs Lake: Size

Seas are larger than lakes in the vast majority of cases. In fact, the largest lake is the Caspian Sea, and it is about 143,000 square miles in size. However, the Baltic Sea, recognized as one of the smaller seas, is 149,000 square miles. Seas can also reach much larger sizes, to the tune of 2 million square miles in the case of the Philippine Sea.

Moreover, seas tend to be deeper than lakes on average, with some of them reaching several hundred feet deep. Also, lakes can be as little as 20ft deep, but they can also range upwards of 5,000ft deep in the case of Lake Baikal.

Still, size isn’t a great way to distinguish these bodies of water. We need more context to define a body of water.

Sea vs Lake: Water Type

Seas are only salt water, but lakes can be freshwater, saltwater, and brackish in very few cases. The vast majority of lakes are freshwater, though. This context helps categorize large bodies of water that could be considered a sea or a lake.  

Sea vs Lake: Boundaries

Lakes are almost completely or entirely landlocked in the case of endorheic lakes, but seas are only partially enclosed by landmasses. Boundaries are one of the most important ways to define a sea or a lake.

Many lakes gain water from rainfall and streams, and they can feed into rivers that flow into the ocean. Seas are bounded by land to some extent, but they are still part of the ocean.

In some cases, like the Sargasso Sea, the sea is bounded by four major ocean currents. This area is distinct due to the calmness of the water and the unique seaweed in the area.  

Sea vs Lake: Formation

Lakes can be manmade or natural, but seas are always natural. Manmade lakes are constructed through a process of damming rivers in the majority of cases. These lakes are often turned into reservoirs to provide drinking water for human settlements.

Sea vs Lake: Current

Seas have a natural current similar to the ocean in that they are driven by wind, salinity, temperature, and more. Most lakes are only affected by wind currents due to their small size and freshwater. However, a few lakes are affected by salinity and temperature changes. Still, the lakes with complex currents are relatively few.

Sea vs Lake: The Case of the Caspian Sea

The Caspian Sea can be considered a lake, a sea, both, or neither.

©Marina Khlybova/

The Caspian Sea is often recognized as the world’s biggest lake. Yet, some people have made a case for this body of water being designated a sea. Using the differences between the two bodies of water, we can evaluate the Caspian Sea and come to a firmer conclusion.

The surface area might make you think it’s a sea, and the salinity might help that case, too. However, the water is not as salty as seawater and is an inland body of water, making the case that it’s a lake.

The Caspian Sea is the perfect body of water to demonstrate the overlap and lack of distinction between lakes and seas. In some cases, it’s open to interpretation, so many people consider the Caspian Sea a sea, and others call it a lake. Officially, it can be either one or even neither.

Few bodies of water are as confounding as the Caspian Sea. However, it’s useful to apply the qualities of a sea vs lake and see how some confusion can still linger.

Why is the Black Sea a Sea and Not a Lake?

This body of water is considered part of the worldwide ocean system. It possesses saline waters, maintains a global sea level, and boasts a bustling maritime route as it connects to the Mediterranean Sea through the Bosporus and Dardanelles at Istanbul, which ranks among the busiest shipping straits on the planet.

The Black Sea serves as a prime illustration of an inland sea. Positioned at sea level, it remains accessible to the ocean.

The reason it often resembles a lake is due to its considerable distance inland and its indirect connection to the ocean.

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About the Author

Kyle Glatz is a writer at A-Z-Animals where his primary focus is on geography and mammals. Kyle has been writing for researching and writing about animals and numerous other topics for 10 years, and he holds a Bachelor's Degree in English and Education from Rowan University. A resident of New Jersey, Kyle enjoys reading, writing, and playing video games.

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