Homebrew ABV Calculator
Calculate the alcohol percentage (ABV) of your homebrew, based on gravity measurements before and after fermentation.
How to use the calculator
- Measure the original gravity (OG) with a hydrometer or refractometer before fermentation starts.
- Measure the final gravity (FG) once fermentation is complete.
- Enter both values to get the alcohol percentage of the brew.
How the alcohol percentage is calculated
OG (Original Gravity) and FG (Final Gravity) are density measurements of the wort before and after fermentation, taken with a hydrometer or refractometer. The difference between them tells you how much sugar the yeast has converted to alcohol:
The more sugar converted to alcohol, the bigger the difference between OG and FG, and the higher the alcohol percentage.
Why the hydrometer reading is rarely quite right
A hydrometer measures the density of the liquid, and density varies with temperature. The instrument is calibrated for one particular temperature — usually around 20 °C — so a reading taken in warm wort shows a lower density than the true value. Measure OG right after the boil while the wort is still lukewarm, and FG at room temperature, and you introduce a systematic bias that feeds straight into the calculated alcohol content.
The other common error is measuring too early. Fermentation can look finished because the airlock has stopped bubbling, without actually being complete. The real test is two identical readings two or three days apart: if the density holds steady, fermentation is over. If it is still moving, the final reading — and therefore the alcohol percentage — is too high.
Frequently asked questions
What are OG and FG?
OG (Original Gravity) is the density of the wort before fermentation, and FG (Final Gravity) is the density after fermentation is complete. Both are usually measured with a hydrometer or refractometer.
How accurate is this formula?
The formula gives a good estimate for most homebrews, but is simplified. Actual alcohol percentage can vary somewhat depending on yeast type, fermentation temperature, and how accurately the measurements were taken.
How do I measure OG and FG at home?
Use a hydrometer (lowered into the liquid) or a refractometer (a few drops on the lens) to read the density. Measure OG right before adding yeast, and FG once fermentation has stabilized over several days.