Great question! In brewing, there are a couple of methods for calculating Alcohol % by Volume. These methods use different formula, numbers and science. Beer Calculators use a method that calculate ABV using PHYSIOLOGY, while Brewmeter uses a method that calculates using CHEMISTRY. Since ABV is a ratio, if we start with 2 numbers, we can find the 3rd. For example, if we start with 2 numbers 5 and 3, then we can determine the 3rd number is 150. Physiology method uses the rules of human body to find out alcohol content. First of all, it assume the weight of your body is average weight of 70kg. In other words, it assumes your weight is 70kg if you started with a weight of 70kg. It assumes the density of your body is 0.8g/ml. Therefore, it assumes your density is 0.8g/ml if you started with a density of 0.8g/ml. It assumes the volume of your body is 80L. In other words, it assumes your volume is 80L if you started with a volume of 80L. Therefore, for your body, Alcohol % by Volume = alcohol content x 0.8g/ml x 80L/70kg = (1 x 0.8g/ml x 80L/70kg) / (0.8g/ml x 80L/70kg) = 0.8. Therefore, final alcohol content by volume.