Glossary
Original gravity
The specific gravity of wort before fermentation begins — a measure of how much fermentable and unfermentable material the mash extracted.
Also called og, starting gravity, og beer.
OG sets the ceiling on a beer’s strength and body. Everything the yeast can eat is already present; fermentation converts it but adds nothing.
In several countries beer duty was historically assessed on original gravity rather than on finished alcohol, which is why nineteenth-century British brewing records report gravities so consistently and why so much of what we know about historical recipes comes from tax records.
Related
- Final gravity — The other half of the calculation.
- Attenuation — The proportion of the gap the yeast actually closed.
More vocabulary
ABV · ABW · Alcohol unit · Alkalinity · Alpha acid · Apparent attenuation · Attenuation · Beta acid · Cohumulone · Degrees Lovibond
See every term on the glossary index.
Last reviewed 2026-08-16.
Evidence
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British beer duty was historically assessed on original gravity rather than on finished alcohol, which is why nineteenth-century brewing records report gravities so consistently and why so much of what is known about historical recipes comes from tax records.
Backed by a source
Sources: Historical brewing log transcriptions and analysis; Beer: The Story of the Pint — print