Original gravity, final gravity and ABV explained
What specific gravity measures, how the 131.25 formula turns two readings into alcohol content, and why attenuation matters.
What specific gravity measures
Specific gravity compares the density of your wort or beer to plain water, which is defined as 1.000. Dissolved sugar makes the liquid denser, so an unfermented wort might read 1.050, meaning it is five percent denser than water. As yeast eats that sugar and produces alcohol and carbon dioxide, the density falls. The two snapshots that matter are original gravity, taken before fermentation, and final gravity, taken once the yeast has finished. The gap between them is the raw material for every calculation this tool performs.
Turning two readings into ABV
The homebrew standard is ABV = (OG - FG) x 131.25. The difference between your readings reflects how much sugar was converted, and the 131.25 factor scales that drop into a percentage of alcohol by volume. A beer that starts at 1.055 and finishes at 1.012 has a gravity drop of 0.043, which multiplies to about 5.6 percent. This linear formula is simple and reliable for everyday beers, and it is the same one printed in most introductory brewing guides.
Reading attenuation
Attenuation tells you how thoroughly the yeast did its job. Apparent attenuation of (OG - FG) / (OG - 1) x 100 expresses the drop as a fraction of the original sugar. A highly attenuative yeast strain and a fermentable wort might reach 80 percent or more, leaving a dry, crisp beer, while a lower figure leaves more residual sweetness and body. If your attenuation comes out much lower than the yeast's published range, it can be a clue that fermentation stalled or needs more time.
Why the calorie figure is only a guide
Alcohol carries roughly seven calories per gram, and residual carbohydrates add more, so a beer's calorie count depends on both its strength and how much unfermented sugar remains. The estimate here scales with ABV alone, which captures the alcohol contribution but not the exact sugar left behind. That makes it useful for comparing one recipe to another, yet it will not match a laboratory nutrition panel. For a precise figure you would need to account for the specific residual extract of your finished beer.