What is the Specific Gravity to ABV Calculator?
This calculator estimates the alcohol content of a fermented beverage from two hydrometer readings: the original gravity (OG) taken before fermentation and the final gravity (FG) taken after. It is widely used by homebrewers and winemakers to gauge the strength of beer, wine, mead, and cider.
How to use it
Take a hydrometer reading of your unfermented wort or must to get the OG (typically 1.030–1.120). After fermentation finishes, take another reading for the FG (often 0.990–1.020). Enter both values and the calculator returns the estimated ABV percentage along with the gravity drop and approximate alcohol by weight.
The formula explained
The popular short formula is $$\text{ABV} \% = \left( \text{OG} - \text{FG} \right) \times 131.25$$ The factor 131.25 converts the density change caused by sugar being turned into ethanol and carbon dioxide into a percentage of alcohol by volume. It is an approximation that is most accurate for typical beer-strength brews; very high-gravity beverages drift slightly from the true value.
Worked example
Suppose your wort started at an OG of 1.050 and finished at an FG of 1.010. The gravity drop is \(1.050 - 1.010 = 0.040\). Multiplying by 131.25 gives $$0.040 \times 131.25 = 5.25\% \text{ ABV}$$ The approximate alcohol by weight is \(5.25 \times 0.79336 \approx 4.17\%\).
FAQ
Why is my reading different from a packaged label? The 131.25 factor is an estimate; commercial labs use more precise distillation methods, and temperature affects hydrometer readings.
Should I correct for temperature? Yes — hydrometers are usually calibrated at 20°C (68°F). Adjust your readings if your sample is significantly warmer or colder for best accuracy.
Can I use this for spirits? No. This formula only applies to fermented beverages before distillation, not to distilled spirits.
