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 had conquered the parishes of Idre and Särna in 1644 lake Femunden was considered to be part of the border between Norway and Sweden. But this was never officially recognised by Norway (or in reality by the Danish government, since Norway was ruled from Copenhagen in the early modern age), and during border adjustments in 1751 the Femundsmarka east of the lake was granted to Norway from Sweden. The new (and current) border from 1751 is quite special: For a length of 61 kilometers (nearly 40 miles) it makes a completely straight line between the summits of Våndsjögusten (1002 meter) and Østerhogna (1185 meter), straight-line national borders being very unusual in Scandinavia except in the very northernmost parts.