Homebrew ABV Calculator

How ABV Is Calculated from Gravity Readings

Every brewer takes two key measurements: Original Gravity (OG) before fermentation starts and Final Gravity (FG) after fermentation finishes. The difference between these two numbers tells you how much sugar the yeast consumed and converted into alcohol and carbon dioxide. The wider the gap between OG and FG, the more alcohol was produced.

The standard formula used by most homebrewers is straightforward: ABV = (OG - FG) × 131.25. This works accurately for beers up to about 6% ABV. For higher-gravity beers like imperial stouts, barleywines, and Belgian strong ales, the alternate formula provides a more accurate reading because it accounts for the changing density of the solution as alcohol content increases. The alternate formula is: ABV = 76.08 × (OG - FG) / (1.775 - OG) × (FG / 0.794).

Specific Gravity vs Plato

Brewers measure the density of wort (unfermented beer) using one of two scales. Specific Gravity (SG) compares the density of the wort to pure water at the same temperature. Water is 1.000, and a typical beer wort before fermentation reads around 1.040 to 1.060. The higher the number, the more dissolved sugar is present.

Plato (°P) measures the percentage of sucrose equivalent dissolved in the wort by weight. A reading of 12°P means the wort has the same density as a solution of 12% sucrose by weight. Professional breweries tend to use Plato because it maps more directly to extract potential and grain bills. Homebrewers more commonly use SG because most entry-level hydrometers are marked in SG. This calculator works with both and converts between them automatically.

Understanding Apparent Attenuation

For quick percentage math on attenuation or efficiency, our percentage calculator handles the arithmetic.

Attenuation tells you what percentage of the available sugar the yeast consumed. An attenuation of 75% means the yeast ate three-quarters of the fermentable sugar. Typical attenuation for most ale yeast strains runs between 72% and 78%. Highly attenuative strains like Belgian saison yeasts can hit 85% to 95%, producing a very dry beer. Low-attenuation strains leave more residual sweetness, resulting in a fuller-bodied beer with lower alcohol relative to the starting gravity.

If your attenuation is unusually low (below 65%), it may indicate a stuck fermentation. Common causes include pitching too little yeast, fermenting at too low a temperature, or a wort with a high proportion of unfermentable dextrins from a high-temperature mash.

IBU Calculation with the Tinseth Formula

International Bitterness Units (IBU) measure the concentration of iso-alpha acids in beer, which are the compounds that give beer its characteristic bitter taste. The Tinseth formula, developed by Glenn Tinseth, is the most widely used method in homebrewing software. It accounts for two key variables: wort gravity (higher gravity reduces hop utilization) and boil time (longer boil times extract more bitterness).

To calculate IBU, the formula needs the weight of hops in grams, the alpha acid percentage of those hops (printed on the package), the boil time in minutes, and the batch volume. This calculator lets you add multiple hop additions, which is how most recipes are structured. A typical pale ale might have a 60-minute bittering addition and a 15-minute flavour addition and a 0 to 5 minute aroma addition. Only the boil additions contribute significantly to IBU; dry hops and whirlpool hops add negligible bitterness.

Calories in Beer

The calorie estimate uses the standard formula based on original and final gravity. Beer calories come from two sources: alcohol itself (which has 7 calories per gram) and residual carbohydrates that the yeast did not ferment. A light lager with 4% ABV typically has around 100 to 120 calories per 355ml (12oz) serving. A 7% IPA runs around 200 to 230 calories. An imperial stout at 10% can exceed 300 calories per serving.

Practical Tips for Accurate Readings

• Calibrate your hydrometer: Hydrometers are calibrated to read correctly at a specific temperature, usually 15°C or 20°C. If your sample is warmer or cooler, apply a temperature correction.
• Degas the sample: CO2 dissolved in fermenting beer artificially lowers hydrometer readings. Let the sample sit for a minute or gently swirl it to release bubbles before reading.
• Read at the meniscus: Read the hydrometer scale at the bottom of the curved surface where the liquid meets the stem, not at the top of the curve.
• Use a refractometer carefully after fermentation: Refractometers give inaccurate post-fermentation readings because alcohol changes the refractive index. Apply a refractometer correction formula or use a hydrometer for FG.
• Take multiple readings: Fermentation is complete when the gravity reading stays the same over two to three days. A single reading does not confirm fermentation is finished.