Hjälmaren is Swedens fourth largest lake. It is situated in the Central Swedish lowland and drains through Eskilstunaån into the adjacent Lake Mälaren, which in turn drains into the Baltic Sea, west of Stockholm. It is connected by waterway with Stockholm by the 13 kilometres long Hjälmare kanal.
It is bounded by the provinces of Södermanland, Närke, and Västmanland.
Hjälmaren is 63 kilometres long, about 20 kilometres wide, and has an average depth of 6 metres, and a surface of 483 km2. The largest towns around the lake is Örebro and Eskilstuna situated at the western and eastern ends of the lake.
