A hydrometer is any piece of equipment that tests the density of a liquid by using the principles of buoyancy and specific gravity. Hydrometers are typically glass with a large bulb at one end that contains a weight, and a narrow stem at the other end with graduated scales to read the measurement. The scale normally used for winemaking in the United States is Brix.

To perform a measurement the hydrometer is floated into the liquid and will sink to a specific level depending on the density of the liquid. Denser liquids are more difficult to displace, so the hydrometer will be more buoyant the denser the liquid becomes. Higher concentrations of dissolved solids like sugar increase the density of a liquid. Density is also a temperature dependant measurement that increases with colder temperatures and decreases at warmer temperatures.

Hydrometers must be calibrated to a standard. Pure water at 20°C is often used as the zero point. In the case of Brix, 1° Brix equals a water solution with 1% of the solution weight as dissolved sucrose at 20°C. Each subsequent degree is an increase by 1 additional percent weight of sucrose.