In more general terms specific gravity is the ratio of the density of a material to that of any standard substance, although usually this is water at 4 degrees Celsius or 39.2 degrees Fahrenheit. By definition, water has a density of 1 kg per litre at this temperature. The specific gravities of gases usually are compared to dry air which generally has a density of 1.29 g per litre. The specific gravity of all other materials is compared to water as a fraction heavier lighter or heavier density.