The Baltic Sea is small, shallow and surrounded by several states

The Baltic Sea is a small sea on the northeastern edge of the Atlantic Ocean surrounded by landmasses in nearly all directions. Its surface area is slightly larger than Finland’s and its mean depth is only 54 metres.


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

     Itämeri on jaettu kuuteentoista alueeseen, joista Suomen rannikolla ovat pohjoisesta alkaen Perämeri, Selkämeri, Ahvenanmeri, Saaristomeri, Pohjoinen Gotlannin allas ja Suomenlahti
    The different sea areas of the Baltic Sea. The division into areas is based partly on the natural basins formed on the seafloor and partly on agreed upon borders. (Source: Myrberg et al. 2006)

    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 depth map oh the Baltic Sea

    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. 

    Sea basin Surface area (km2) Mean depth (m) Deepest depth (m) Volume (km3)
    Kattegat 22,287 23 130 515
    the Baltic Sea (with danish straits) 392,978 54 459 21,205
    the Baltic Sea (without danish straits) 372,857 56 459 20,918
    Danish straits 20,121 14 81 287
    Belt sea 17,821 15 81 260
    Öresund 2,300 12 53 27
    Arkona basin 19,068 23 53 442
    Bornholm basin 38,942 46 105 1,780
    Gotland basin 151,920 71 459 10,824
    Bay of Gdansk 25,234 57 114 1,439
    Eastern Gotland basin 63,478 77 249 4,911
    Northern Gotland basin 28,976 71 150 2,056
    Western Gotland basin 34,232 71 459 2,418
    Gulf of Finland 29,498 37 123 1,098
    Gulf of Riga 17,913 23 51 405
    Gulf of Bothnia 115,516 55 293 6,369
    Sea of Åland 5,477 75 301 411
    Archipelago Sea 8,893 19 104 169
    Bothnian Sea 64,886 66 293 4,308
    Bothnian bay 36,260 41 146 1,481

    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.