Typically, for about every 1 ft2 of south-facing glass, about 5 to 10 ft3 of thermal mass is required for thermal mass (1 m3 per 5 to 10 m2). When accounting for minimal-to-average wall and floor coverings and furniture, this typically equates to about 5 to 10 ft2 per ft2 (5 to 10 m2 per m2) of south-facing glass, depending upon whether the sunlight strikes the surface directly. The simplest rule of thumb is that thermal mass area should have an area of 5 to 10 times the surface area of the direct-gain collector (glass) area.