Final gravity (FG)

Final gravity, or FG, is the density of your beer once yeast has finished eating available sugars and fermentation has settled down. It is always lower than original gravity because sugar has been converted into alcohol and carbon dioxide, and alcohol is less dense than the sugar solution it replaced. Where OG tells you what you started with, FG tells you where the beer landed.

You measure FG the same way as OG, with a hydrometer or a properly corrected refractometer, but you take the reading only after gravity has held steady across two or three days rather than after a set number of days on the calendar. Fermentation speed depends on yeast strain, temperature, and the sugars in your particular wort, so watching the number stabilize is more reliable than guessing based on time alone.

The gap between OG and FG is what feeds an ABV formula, so an inaccurate FG throws off your alcohol estimate. A frequent error is bottling or kegging too early because gravity looks close to the recipe's target, when it is actually still slowly dropping. Take two readings a few days apart, and if they match, fermentation is genuinely finished rather than just paused.