Original Gravity (OG), sometimes called original extract, is a measure of the solids content originally in the wort, before alcoholic fermentation has commenced to produce the beer. OG is one of the major measurements used by brewers to determine the future alcohol content of a beer fermented from a particular wort. It is measured by a saccharometer, hydrometer, or refractometer as the density of the wort at standard temperature and pressure (STP; usually 20°C and 760 mm) at the final collection point before the yeast is added. Original gravity is expressed as the density above that of distilled water and in the UK is called the excess gravity. Water is deemed to have a density at STP of 1.000. If the wort density is 1.048, it will have 48° of excess gravity and an OG of 48.
Internationally, different units are used to express OG that are unique to the brewing industry and include degrees Plato, degrees Balling, or percent dry matter of the wort, Brix % (for sucrose only). These units take into account the solution factors of carbohydrates and mixtures of carbohydrates typically found in wort made from different cereal/malt recipes (e.g., barley malt, maize, rice, sugar). The numerical figure for these units approximates one-quarter of the excess gravity. In the example above 48/4 = 12% dry matter by weight or 12° Balling or 12° Plato.
Laboratory procedures can be used to establish the original gravity of a beer by measuring both the present or apparent specific gravity of the beer and the alcohol content of the beer, the latter by distillation. Original gravity tables convert the alcohol content back to the amount of carbohydrate fermented to produce it. Then, by the addition of these two values, the wort’s original gravity can be established from the tables. Tables for converting alcohol content to a value for carbohydrate fermented were first produced in 1850 by Graham, Hofman, and Redwood and were incorporated into the UK 1880 Inland Revenue Act for excise duty collection calculations as Statutory Tables. They were found to give inaccurate results under certain circumstances (Section 15 of the Act Export drawback, etc.) so in 1910 after a Joint Inquiry led by Sir T. E. Thorpe and Dr H. T. Brown of the Institute of Brewing, they were refined to accord with “brewing operations as carried out in present day brewery practice.” The use of high-gravity brewing technology and postfermentation dilution is ignored by this analysis because it measures the OG as if no dilution had occurred.
Brewers seek to achieve consistent original gravities for their worts as part of overall quality assurance for their beers.