What does a hydrometer measure?
In beer-making terms, a hydrometer tells you how much sugar, and other stuff, is in your beer. However, all you’re interested in is sugar. As the sugar gets eaten by the yeast and turned into alcohol, the hydrometer drops because sugar is way heavier than water, while alcohol is just a little bit lighter. When the hydrometer stops dropping for 3-4 days straight, fermentation is complete (as the yeast is no longer turning sugar in to booze) and you can either bottle or rack. There are no appropriate numbers, as every beer is different. I’ve had beers with starting gravities above 1.10 (imperial stout) and lower than 1.45 (bohemian pilsner) with finishing gravities anywhere from 1.025 (very sweet and heavy) to 1.005 (ultra-light). Just ferment until it finishes.
