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<table align=right><td>[[imageFile:Hydrometer.png|right|thumb|Hydrometer]]</td></table>A '''hydrometer''' is a device used to measure the specific gravity of unfermented wort and fermented beer during the [[Processes|brewing process]]. The most common form of brewing hydrometer is a hollow glass device with a metal weight at the bottom. A paper scale is included inside the tube, and usually marked in one or more scales. The hydrometer floats freely in the liquid you are measuring, and is read at the point where the surface of the liquid touches the stem. The most frequently used scale on a hydrometer for brewing is the specific gravity scale. The [[specific gravity ]] scale provides a relative measure of sugar as compared to water. Water would have a specific gravity of 1.000, while most finished beers have a starting gravity before fermentation of 1.030-1.060 and around 1.010 is typically expressed to three digits after fermentation. Sometimes specific gravity is shown as a 4 digit number without the decimal point - for example as in ''10401.040''. Fermentation lowers the specific gravity of the beer because alcohol is less dense than water.

The most common type of hydrometer sold for brewing is calibrated for use on water at temperature of 60 F. If the beer liquid you are measuring is at a different temperature, you need to make an adjustmentto account for the difference between calibration temperature and the liquid's temperature. You can perform this calculation with a brewing calculator such as [http://www.beersmith.com BeerSmith] or you can use a hydrometer adjustment table such as [http://www.ebrew.com/primarynews/ct_hydrometer_temp_correction.htm this one].