Hydrometers in general use may be divided into three classes with reference to their indication: 1. Specific gravity hydrometers; 2. per cent hydrometers; 3. arbitrary scale hydrometers.

Specific gravity hydrometers indicate the ratio of the weight of a given volume of the substance to the weight of the same volume of some standard substance. The standard substance is usually water at a definite temperature.

Per cent hydrometers indicate the percentage of a substance, either by weight or by volume, in a mixture or solution of the substance in water.

Arbitrary scale hydrometers indicate the concentration or strength of a substance in terms of some arbitrarily defined scale. Lactometers and Baume hydrometers are examples of this class.