A knowledge of the density or specific gravity of a liquid is usually of value in the household only as an index of some other physical property or quality of the liquid. For example, in the preparation of sirups, jellies, and other food products of similar nature, a measurement of specific gravity is a convenient means of determining when the process of evaporation or "boiling down" has been carried far enough. Also, the quality or fat-content of milk may be determined by measuring its specific gravity.

Specific gravity is the ratio of the weight of any volume of a substance to the weight of an equal volume of water. Milk has a specific gravity of 1.03, since the weight of any volume of milk is 3 per cent more than that of an equal volume of water. The densities given in Table IV are stated in grams to the cubic centimeter, and are numerically the same as specific gravity in terms of water at 4° C. as unity.