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HOW TO CALCULATE ABV As well as indicating when fermentation is complete gravity readings can be used to calculate how much sugar has been converted to alcohol, and so determine the strength of your brew. You will need to take a reading before pitching the yeast (the original gravity or OG) and a second reading before bottling or barreling (the final gravity or FG). Multiplying the difference between these two readings by 105 will give you the percentage of alcohol by weight. To determine the percentage of alcohol by volume (ABV)- the measure used by most commercial brewers-simply multiply the alcohol-by-weight figure by 1.25.