From Bryce Eddings,
Your Guide to Beer.
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Definition: OG - Original gravity is a measure of the fermentable and unfermentable substances in the wort before fermentation. OG gives the brewer an idea of the potential alcoholic strength of the final product. After fermentation when OG is mathematically compared to FG, Final Gravity, the exact alcoholic strength can be determined.

Examples: Kyle's Nut Brown ale had an OG of 1.054 and an FG of 1.015 resulting in 5.3% alcohol by volume.