How To Read A Hydrometer. By taking a specific gravity reading before the introduction of your yeast to the must, then again when fermentation is complete can you determine the alcohol percentage by volume. Subtract the final reading from the beginning to get your answer. When your hydrometer stops declining for 3 days in a row can you be certain that fermentation has ceased. Hydrometers can contain any or all of the following scales, specific gravity, % of alcohol by vol., pounds/ounces of sugar per gallon and temperature.