Hydrometers are calibrated glass floats that are used to compare the densities of liquids. They are used to measure the specific gravity (SG) or relative density of a material. Specific gravity is the density of a material divided by the density of water at standard temperatures. Specific gravity is expressed as a ratio. Specific gravity instruments include Baumé hydrometers for liquids, such as syrups, that denser than water; salt hydrometers, which show the percentage of salt in solution by weight; Brix hydrometers, devices used by the sugar industry to indicate the percentage of sucrose by weight; API hydrometers, which use a standard scale for petroleum products as defined by American Petroleum Institute (API); and Tralle and proof hydrometers, instruments which display the percentage of ethyl alcohol by weight in water. A thermohydrometer includes both a thermometer and a hydrometer. Thermohydrometers enable the accurate measurement of both temperature and specific gravity of a sample.

Accurate water density measurement can be important to the accuracy of other measurements. For example, to calibrate flow meters accurately, hydrometers are used to measure the specific gravity of the water flowing in the test facility. Calibration of hydrometers can be performed by an organization such as the National Institute of Standards and Technology (NIST), or by a calibration laboratory accredited by ISO/IEC 17025. Calibration methods for hydrometers vary according to hydrometer design. For example, volumetric calibration is used for burets, pipets, volumetric flasks, graduated cylinders, and other volumetric glassware.

Hydrometers conform to requirements and standards established by several different organizations. ASTM International (formerly called the American Society for Testing and Materials (ASTM) maintains ASTM specifications E-100 (Standard Specification for ASTM Hydrometers) and E-126 (Standard Test Method for Inspection and Verification of Hydrometers). Hydrometers are also covered by standards published by The International Organization for Standardization (ISO) Technical Committee 28, Subcommittee 3 (static petroleum measurement).