The 204-square-kilometre (79 sq mi) lake holds about 6 cubic kilometres (1.4 cu mi) of water and reaches a maximum depth of 130 metres (430 ft). The surface of the lake sits about 662 metres (2,172 ft) above sea level.
After Sweden conquered the parishes of Idre and Särna in 1644 the lake Femunden was considered to be the border between Norway and Sweden. But this was never officially recognised by Norway, and in 1751 was Femundsmarka on the east side of the lake granted was to Norway from Sweden. The new (and current) border from 1751 is quite special: For a length of 61 kilometers it makes a completely straight line between the mountaintops of Våndsjögusten (1002 meter) and Østerhogna (1185 meter).