Gravity (beer)

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Original gravity (OG) usually refers to the density of wort, unfermented beer, usually expressed as a ratio to the density of water (thus for instance 1.050, occasionally expressed 1050). The density is closely related to the amount of fermentable sugar dissolved in the wort, and thus predicts the likely strength of the finished beer.

Final gravity (FG) is the density of the beer once the fermentation has finished. The difference between OG and FG is a measure of the amount of sugar consumed in the fermentation, and therefore of the alcohol evolved. Different yeasts have different powers of attenuation (ability to consume wort sugars), and different worts may have higher or lower proportions of non-fermentable sugars, so final gravity is not a simple function of original gravity.

The main reason the density drops during fermentation is that the process breaks down sugar molecules into ethanol and carbon dioxide, the majority of the latter escaping as gas.

An approximate calculation gives the percent alcohol by volume as a function of the original and final gravities as follows:

[ABV=\frac], for example [\frac=5.42\% ABV]

Gravity (beer)

See also