ABV Calculator – Alcohol by Volume
Estimate the alcohol content of your homebrew from two hydrometer readings. Enter the original gravity (OG) and final gravity (FG) to get ABV, apparent attenuation and alcohol by weight.
Estimate Alcohol Content from Gravity Readings
Brewing ABV Formula
Where:
- \(OG\) — original gravity, the hydrometer reading before fermentation (e.g. 1.050)
- \(FG\) — final gravity, the hydrometer reading after fermentation (e.g. 1.010)
- \(131.25\) — an empirical constant that converts the gravity drop into percent alcohol by volume
- \(0.789\) — the specific gravity of pure ethanol, used to convert ABV to alcohol by weight
The first formula is the standard brewing equation: as yeast converts sugar to alcohol, the specific gravity of the wort drops, and the size of that drop is proportional to the alcohol produced.
Brewing ABV – Calculation Example
Example: a batch of ale has an original gravity of 1.050 and finishes fermentation at a final gravity of 1.010.
\[ ABV = (1.050 - 1.010) \times 131.25 = 0.040 \times 131.25 = 5.25\% \]
\[ AA = \frac{1.050 - 1.010}{1.050 - 1} \times 100 = \frac{0.040}{0.050} \times 100 = 80\% \]
\[ ABW \approx \frac{5.25 \times 1.010}{0.789} \approx 6.72\% \]
Result: approximately 5.25% ABV, 80% apparent attenuation and 6.72% alcohol by weight — a typical-strength, well-attenuated ale.
How to Use the Brewing ABV Calculator
- Take a hydrometer (or refractometer) reading of your wort before pitching yeast and enter it in the Original gravity (OG) field, for example 1.050.
- Take a second reading once fermentation has finished — when the gravity is stable for two or three days — and enter it in the Final gravity (FG) field, for example 1.010.
- Read your results: Estimated alcohol by volume, Apparent attenuation and Approximate alcohol by weight, each with a short interpretation note.
Both readings must be taken at the same temperature, or corrected for temperature, since hydrometers are calibrated to a specific reference temperature (usually 20 °C / 68 °F).
What ABV, Attenuation and ABW Mean
Alcohol by volume (ABV) tells you what percentage of the finished drink's volume is pure ethanol. Most everyday beers sit between 4% and 6% ABV, session beers below about 4%, and strong styles such as imperial stouts or barleywines above 9%. Knowing your ABV helps you hit a target recipe, comply with labelling rules, and gauge how strong each serving actually is.
Apparent attenuation describes how much of the fermentable extract the yeast consumed, measured as the relative drop in gravity. It is "apparent" because the alcohol formed is lighter than the sugar it replaced, so the true attenuation is slightly higher. Most ale and lager yeasts attenuate 65–85%. A lower value usually means a sweeter, fuller-bodied beer or an incomplete fermentation; a higher value points to a very dry finish, typical of wine and cider yeasts.
Alcohol by weight (ABW) expresses alcohol as a percentage of the drink's mass rather than its volume. Because ethanol is lighter than water, ABW is always lower than ABV for the same brew. In the United States, beer strength was traditionally quoted in ABW, which is why some older labels show smaller numbers than the ABV you calculate.
Accuracy, Assumptions and Practical Tips
- The formula \( ABV = (OG - FG) \times 131.25 \) is accurate to within a few tenths of a percent for beers up to about 8% ABV. For very strong brews, it slightly overestimates the alcohol; dedicated high-gracity formulas exist for barleywines and imperial styles.
- The calculator assumes a normal sugar-to-alcohol fermentation. Fermenting with fruit, honey or unfermentable additives (lactose, maltodextrin) shifts the relationship between gravity drop and alcohol.
- Always degas and let samples reach the calibration temperature before reading a hydrometer. Carbonation bubbles and temperature off by several degrees can each distort a reading by 0.001–0.003 gravity points, which changes the ABV estimate noticeably.
- If FG equals OG, no fermentation took place; if FG is above 1.020 with an average yeast, fermentation may be stuck — check yeast health and temperature before bottling.
- If you measure sweetness with a refractometer in Brix, first convert your readings with the Brix to specific gravity converter, then use the results here.
- Since specific gravity is a ratio of densities, you may also find the density units converter useful when working with hydrometer scales in kg/m³ or g/cm³.
Frequently Asked Questions
How do I calculate ABV by hand without a calculator?
Subtract your final gravity from your original gravity and multiply by 131.25. For example, with OG 1.048 and FG 1.012: \((1.048 - 1.012) \times 131.25 = 4.73\%\) ABV. If you prefer degrees Plato, the shortcut \(ABV \approx (OG - FG) \times 131\) works just as well for everyday beers. Keep at least three decimal places in the gravity readings, since rounding to 1.05 vs 1.050 shifts the result noticeably.
Why is my calculated ABV different from what a brewery or lab reports?
The standard formula is an approximation that drifts at higher alcohol levels. For brews above about 8% ABV, the alternate formula \(ABV = (76.08 \times (OG - FG) / (1.775 - OG)) \times (FG / 0.794)\) is more accurate. Labs also use distillation or gas chromatography, which measure ethanol directly rather than inferring it from gravity. Expect your result to be within 0.2–0.5% of a lab value for normal-strength beer.
Can I use a refractometer instead of a hydrometer?
Yes before fermentation, but not directly after. Alcohol bends light differently than sugar, so a post-fermentation refractometer reading in Brix is misleading on its own. You can still get ABV by combining the original Brix reading with the final refractometer reading using a special correction formula, or simply use a cheap hydrometer for the final reading — it stays accurate in the presence of alcohol.
Does this formula work for wine, cider and mead?
The gravity-drop method works for any fermented drink, but accuracy varies. Wines and ciders often start at OG 1.090–1.110 and finish near 0.990–1.000, where the simple formula starts to overestimate alcohol by up to a percentage point or more, so a high-gravity formula is recommended. Mead and fruit ferments also contain non-sugar extract that affects the readings, so treat the result as a close estimate rather than an exact figure.
How much does temperature affect my hydrometer reading?
A sample warmer than the calibration temperature (usually 20 °C / 68 °F) reads too low, and a colder sample reads too high. A reading taken at 30 °C can be off by roughly 0.002 gravity points — enough to change the ABV estimate by about 0.25%. Most hydrometers ship with a correction table; alternatively, cool or warm the sample to the calibration temperature before measuring. Always use the same temperature for both readings.
What does it mean if my final gravity is higher than expected?
An FG stuck above roughly 1.020 with an average yeast usually signals an incomplete fermentation — common causes are yeast that is too old or under-pitched, temperatures outside the yeast's preferred range, or lack of fermentable sugars. Before bottling, confirm the gravity is stable for two to three days; bottling an actively fermenting beer creates over-carbonation and possible bottle bombs. Rousing the yeast or raising the temperature a few degrees often restarts a stalled fermentation.
What is the difference between ABV, ABW and proof?
ABV is alcohol as a share of volume, ABW as a share of mass — ABW is always lower because ethanol is lighter than water. Proof is mainly a US label convention: US proof equals twice the ABV (80 proof = 40% ABV), while UK proof historically used a different 175% reference. When comparing drinks or tracking your intake, rely on ABV, since serving guidelines and nutrition labels use that measure.