Lake Mjoavatn has an area of 0,75 km². Its greatest depth is 2,5 m and it is situated 208 m above sea level.
Its discharge, Mjoavatnslaekur, connects it with lake Stiflisdalsvatns. During dry spells, the road to the lake is passable by car. The lake is surrounded by bogs. The catch is mainly brown trout, 1-3 pounders. The lake is not netted.
The distance from the capital is about 35 km.