How to use the abv calculator for homebrewing
- Measure the original gravity (OG) with a hydrometer or refractometer before you pitch the yeast and write it down, for example 1.050.
- When fermentation has finished, take the final gravity (FG) reading. It is normally much lower, such as 1.010.
- Enter both readings. Open the advanced options if you want to change the serving size used for standard drinks.
- Read the alcohol by volume, the second formula for comparison, the apparent attenuation and the standard drink count.
Formula
Standard formula: ABV = (OG - FG) x 131.25
Advanced formula: ABV = 76.08 x (OG - FG) / (1.775 - OG) x (FG / 0.794)
Apparent attenuation = (OG - FG) / (OG - 1) x 100
US standard drinks = fluid ounces x ABV / 100 / 0.6
The standard formula is a straight line fitted to typical beer and is the one most homebrewers use. The advanced formula bends with gravity and agrees with it for ordinary beers but drifts higher for strong ones. A standard drink in the United States holds 0.6 fluid ounces of pure alcohol.
Worked example
A pale ale starts at OG 1.050 and finishes at FG 1.010. The gravity drop is 0.040, so the standard formula gives 0.040 x 131.25 = 5.25% ABV.
The advanced formula gives 76.08 x 0.040 / 0.725 x (1.010 / 0.794) = 5.34% ABV, about a tenth of a point higher.
Apparent attenuation is 0.040 / 0.050 = 80%. A 12 fl oz serving at 5.25% holds 0.63 fl oz of alcohol, which is 1.05 standard drinks.
Why two ABV numbers are shown
Alcohol is produced when yeast turns sugar into ethanol and carbon dioxide, and ethanol is lighter than water. That is why the drop in gravity tracks the alcohol made. No single straight line fits every recipe, so brewers use more than one formula.
For beers under about 7 percent the two results are close and either is fine. For strong ales, barleywines, meads and ciders, compare both and treat the difference as the uncertainty in your own reading.
Getting good gravity readings
The result is only as good as the two readings. Small mistakes in gravity make visible changes in ABV, because each 0.001 of gravity is worth about 0.13 percentage points.
- Cool the sample to the temperature printed on your hydrometer, or correct the reading for temperature.
- Remove yeast and large bubbles from the sample, and spin the hydrometer to release gas that clings to it.
- Take FG on two days in a row. If the number has not moved, fermentation is done.
- Read the bottom of the curved surface at eye level, and a refractometer needs a correction once alcohol is present.
Assumptions and limits
- Gravity readings are specific gravity values such as 1.050, not Plato or Brix. Convert those first.
- The formulas assume the gravity drop is caused by fermentation. Added sugar, fruit or lactose after the first reading change the result.
- The standard drink count uses the United States definition of 0.6 fluid ounces of pure alcohol. Other countries use other sizes.
- This is an estimate for hobby use and not a lab measurement or legal labeling figure.
Frequently asked questions
What is a good original gravity for beer?
Most standard beers start between about 1.040 and 1.060. Light lagers sit near the bottom of that range, while stouts and strong ales start higher. The right number depends on the style and the recipe you are following.
Why is my ABV different from the recipe?
Recipes predict a final gravity, but real fermentation depends on yeast health, temperature and mash conditions. If your FG ended higher or lower than the recipe says, your ABV moves with it. Your own measured readings are the ones to trust.
Is apparent attenuation the same as real attenuation?
No. Apparent attenuation uses the hydrometer reading as it is. Alcohol is lighter than water and makes the reading look lower than the sugar left would suggest, so real attenuation is a few points lower than the apparent figure.
Can I use this for wine, cider or mead?
The same readings work, and many people use them. Those drinks often reach gravities below 1.000 and much higher alcohol, where the two formulas can disagree by a full point. Compare both results rather than relying on one.