A hydrometer measures the weight of a liquid relative to the same volume of water (i.e., relative densities). In brewing, much of this excess weight is expected to be from fermentable and unfermentable malt sugars. Most hydrometers measure Specific Gravity (SG), which tells how many times heavier than water the liquid of interest is; for example, a 1.050 SG wort is 1.05 times heavier than an equal volume of water at 60 F. SG measurements are temperature dependent, and SG should be measured at 60 F., as water is SG 1.0 at 60 F.

Hydrometers often come with a temperature conversion chart, but hydrometers often are not accurately calibrated, so that water at 60F will not read 1.0. An easy way to take SG readings with a hydrometer is to measure at room temperature, and then measure water at room temperature and take the difference.

Some abbreviations commonly used in homebrewing relating to specific gravity: OG, Original (wort specific) Gravity; FG or TG, Final or Terminal Gravity (when the beer is finished fermenting).

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