Alcohol content can be calculated by measuring the change in the specific gravity (SG) during the fermentation process. For reference, water has an SG of 1.000 at 15.5C or 60F. If the corrected SG of the wort at the start of the fermentation process is 1.045 and 1.010 at the end of fermentation, the SG has dropped by 0.035. This is also referred to as a drop in gravity of 35. This drop is caused by the conversion of sugars to alcohol. The alcohol content can be approximated by dividing the drop in gravity by 7.4. Therefore, a drop of 35 gravity means the alcohol content is approximately 35/7.4 or 4.7% alcohol by volume. While this is a decent rule of thumb, it should be noted that the factor by which the gravity change is divided is not a constant. A value of 7.4 is only accurate for a change in gravity of 93, which is much higher than that encountered while brewing beer. A more accurate result can be obtained by calculating what the factor should be and using this value in the alcohol calculation. The factor, F, can be found using: F = 7.75 - (3G/800), where: G = change in gravity. For the example, above, F is 7.75 - ((3x35)/800) = 7.62. The alcohol content, therefore, is: 35/7.62 = 4.6% Note that all SG measurements must be corrected to 15.5C / 60F, using the following correction factors: 10C / 50F = -0.001 15C / 60F = 0.000 20C / 68F = +0.001 25C / 77F = +0.002 30C / 86F = +0.003 35C / 95F = +0.004 40C / 104F = +0.005 Therefore, if the SG of the must is 1.040 at a temperature of 95F, the corrected SG is 1.040+0.004 or 1.044. [With information from "Progressive Winemaking", Duncan and Acton.]