Homebrew OG/FG Calculator

Original Gravity (OG) is a measure of the sugar content dissolved in your wort before fermentation begins. It tells you how much fermentable material is available for yeast to convert into alcohol and CO2. A higher OG means more potential alcohol and fuller body in the finished beer.

Final Gravity (FG) is the gravity reading taken after fermentation has finished and the yeast has consumed most of the available sugars. You should measure FG when activity in your airlock has stopped and two readings taken 24–48 hours apart are identical, confirming fermentation is complete.

The standard formula is ABV = (OG − FG) × 131.25. The advanced Miller/Cutaia formula accounts for the changing density of the wort during fermentation and is slightly more accurate for higher-gravity beers: ABV = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794).

Apparent attenuation is the percentage of sugars that yeast has consumed during fermentation, calculated as ((OG − FG) / (OG − 1.0)) × 100. Most ale and lager yeast strains achieve 65–80% apparent attenuation. Lower attenuation results in a fuller, sweeter beer, while higher attenuation produces a drier finish.

Specific Gravity (SG) measures the density of a liquid relative to water (e.g. 1.050), while Plato (°P) expresses the weight percentage of dissolved sugars directly (e.g. 12.4 °P). Professional breweries typically use Plato, while homebrewers often use SG. This calculator supports both — select your preferred unit and enter readings accordingly.

The Standard formula (ABV = (OG − FG) × 131.25) is accurate for most beers in the normal gravity range (OG 1.040–1.080). The Advanced (Miller/Cutaia) formula is better suited for higher-gravity beers (OG above 1.080) such as imperial stouts or barleywines, where the standard formula can slightly underestimate alcohol content.

Beer calories come from both alcohol and residual carbohydrates. This calculator estimates calories per 12 oz (355 ml) using the relationship between gravity readings and the known caloric values of ethanol (~7 kcal/g) and unfermented sugars (~4 kcal/g). Lighter beers with lower OG and higher attenuation will naturally have fewer calories.

A higher-than-expected FG could mean fermentation is not yet complete, the yeast was underpitched or stressed, fermentation temperature was too low, or the recipe contained a high proportion of unfermentable (dextrin) sugars. Always confirm fermentation is truly finished with two stable readings before packaging to avoid over-carbonation.