Surface area, volume and depth

The Baltic Sea’s surface area is around 392,000 square kilometres. The surface area of Finland is around 338,000 square kilometres. This means that the Baltic Sea is slightly larger than Finland.

The Baltic Sea’s volume is around 21,000 cubic kilometres. The surface area and volume of the Baltic Sea are less than one percent of the surface area and volume of the Atlantic Ocean.

The Baltic Sea is only 54 metres deep on average, which makes it a very shallow sea. For example, the average depth of the Mediterranean Sea is around 1,500 metres, while the great oceans average 3,700 metres.

Catchment area

The Baltic Sea has a very large catchment area. A catchment area is the area from which a sea gets its water. The Baltic Sea’s catchment area is over 1,600,000 square kilometres, which makes it four times as large as the sea itself.

The catchment area being so large means that the area from which rivers export nutrients into the Baltic Sea is large as well. This is one of the fundamental reasons the Baltic Sea is susceptible to environmental load.

Basins and sills

The Danish straits

The Baltic Sea is connected to the world ocean only by the narrow Danish straits, which connect the Baltic Sea to the North Sea. The border between the Baltic Sea and the North Sea is located past the Danish straits where the Kattegat begins.

The Danish straits are formed by two straits immediately next to each other: the Öresund strait on the coast of Sweden and the Belt Sea on the coast of Denmark, which is further divided into the Great Belt and Little Belt. The straits are narrow and shallow, which restricts the transfer of water between the North Sea and the Baltic Sea.

The Öresund strait in particular is very shallow. The Drogden waterways on Denmark’s side and the Flintrännan waterway on Sweden’s side are only 7–8 metres deep.

The most critical area in terms of water transfer is the Belt Sea, located south of the Danish straits. Located on the seafloor of the Belt Sea is the Darss Sill, which also determines the maximum draft of ships looking to sail on the Baltic Sea.

The Arkona Basin, Bornholm Basin, Bay of Gdańsk and Gotland Basin

After the Danish straits, the next areas of the Baltic Sea are the Arkona Basin – located between Sweden and Germany – and the Bornholm Basin on the northeastern side of the island Bornholm.

After these comes the Gotland Basin in the east and north. The Gotland Basin is sub-divided into the Eastern and Western Gotland Basins and the Bay of Gdańsk in the south.

The Baltic Proper, Gulf of Finland, Gulf of Riga and Gulf of Bothnia

The area of sea reaching from the Arkona Basin to the northern edge of the Gotland Basin is often referred to as the Baltic Proper or, alternatively, the main basin of the Baltic Sea. Two gulfs are located east of the Baltic Proper, the Gulf of Finland and the Gulf of Riga. In the north, there is also the Gulf of Bothnia.

The Gulf of Finland is a long gulf reaching from east to west. It is the easternmost arm of the Baltic Sea and it has no sea floor barriers, so-called sills, that restrict water circulation.

The Bay of Riga is located in the sea areas of Estonia and Latvia, and it is partially separated from the rest of the Baltic Sea.

In terms of geography, the Gulf of Bothnia is the northernmost basin on the Baltic Sea, and it is also quite separated from the rest of the sea. The Gulf of Bothnia, reaching from south to north, has a large surface area, and it is subdivided into the Bothnian Bay, Bothnian Sea, Sea of Åland and Archipelago Sea.

Main basin of the Baltic Sea and its deepest point

The large area covered by the Arkona Basin, Bornholm Basin and Gotland Basin can be called the main basin of the Baltic Sea. The depth of the main basin varies between 50 and 100 metres, with the shallowest parts being located in the south. Some areas are over 200 metres deep.

The deepest point of the Baltic Sea is in the Western Gotland Basin. It is called the Landsort Deep, and it is 459 metres deep.

The sea areas closest to Finland are significantly less deep. The deepest point of the Bothnian Bay is 146 metres deep, and the deepest point of the Bothnian Sea is 293 metres deep. These deeps are located nearer to Sweden than Finland.

The Sea of Åland is located on a fault zone, so its seafloor is very rugged. The Sea of Åland is a strait connecting Sweden and the Åland Islands, and in the waters near the Märket lighthouse, water depth reaches below 300 metres. The exact depth is all of 301 metres. This is the only point in the Baltic Sea where such depths are reached so close to the coast.

The deepest point in the Gulf of Finland is 123 metres deep and located north of Paldiski, a town in Estonia. 

Characteristics of the Baltic Sea by area. From left to right: surface area, average depth, maximum depth and volume. (Source: Myrberg et al. 2006)

Myrberg, K., Leppäranta, M., and Kuosa, H. 2006. Itämeren fysiikka, tila ja tulevaisuus. Palmenia-sarja 17, Yliopistopaino,  202 s.