Lake Stiflisdalsvatn has an area of 1,65 km². Its greatest depth is 30 m and it is situated 178 m above sea level.
The rivers Kjalka and Mjoadalslaekur and other brooks feed it and its discharge is the fine salmon river Laxa í Kjos. The lake is near road no. 36. It abounds in small brown trout. The lake was mainly netted in the past.
The distance from the capital is about 35 km.