Mels has an area, as of 2006, of 139.2 km² (53.7 sq mi).
 Of this area, 42.6% is used for agricultural purposes, while 29.3% is forested.
 Of the rest of the land, 2.6% is settled (buildings or roads) and the remainder (25.5%) is non-productive (rivers or lakes).

The municipality is located in the Sarganserland Wahlkreis.
 In terms of area, it is the largest municipality in the canton and covers the Weisstannen valley and a part of the Seez valley.
 Northwest of town is Castels hill and west of the Nidberg is Castels fortress.
 It consists of the village of Mels, the settlements of Heiligkreuz bei Mels, Plons, Ragnatsch, the hamlets of Butz, St. Martin, Mädris and Tils, the alpine settlement of Vermol as well as the Walser villages of Schwendi and Weisstannen.

Chapfensee is a reservoir located in the municipality.
Mels has an area, as of 2006, of 139.2 km² (53.7 sq mi).  Of this area, 42.6% is used for agricultural purposes, while 29.3% is forested.  Of the rest of the land, 2.6% is settled (buildings or roads) and the remainder (25.5%) is non-productive (rivers or lakes).
The municipality is located in the Sarganserland Wahlkreis.  In terms of area, it is the largest municipality in the canton and covers the Weisstannen valley and a part of the Seez valley.  Northwest of town is Castels hill and west of the Nidberg is Castels fortress.  It consists of the village of Mels, the settlements of Heiligkreuz bei Mels, Plons, Ragnatsch, the hamlets of Butz, St. Martin, Mädris and Tils, the alpine settlement of Vermol as well as the Walser villages of Schwendi and Weisstannen.
Chapfensee is a reservoir located in the municipality.
Mels