The Baltic sea is about 1600 km long, an average of 193 km wide, and an average of 55 m deep. The maximum depth is 459 m (1506 ft), on the Swedish side of the center.
On the long-term average, the Baltic Sea is ice-covered for about 45% of its surface area at the maximum annually. The ice-covered area during such a typical winter includes the Gulf of Bothnia, the Gulf of Finland, Gulf of Riga, Väinameri in the Estonian archipelago, the Stockholm archipelago and the Archipelago Sea. The remainder of the Baltic itself does not freeze during a normal winter, with the exception of sheltered bays and shallow lagoons such as the Curonian Lagoon. The ice reaches its maximum extent in February or March; typical ice thickness in the northernmost areas in the Bothnian Bay, the northern basin of the Gulf of Bothnia, is about 70 cm (28 in) for landfast sea ice. The thickness decreases farther south.
Freezing begins in the northern extremities of Gulf of Bothnia typically in middle of November, reaching the open waters of Bothnian Bay in early January. The Bothnian Sea, the basin south Kvarken, freezes on average in late February. The Gulf of Finland and the Gulf of Riga freeze typically in late January. In 2011, the Gulf of Finland was completely frozen on 15 February.