Want to calculate the alcohol percentage of home brew? Our ABV calculator calculates the the alcohol percentage of your home brewed beer. The ABV calculator also works out Plato, real extract, apparent and real attenuation, alcohol by weight (ABW), and the calories in your beer.
Just put the original specific gravity and the final specific gravity of your beer below and hit enter.
If you want to learn about the formula to calculate the alcohol percentage of home brew, scroll below the calculator. The full explanation is there!
Calculating ABV yourself
The above calculator is really handy, but it’s nice to know the maths behind it all. Firstly, it’s a little more fun that way, but learning the maths helps to understand what’s actually going on calculating the alcohol percentage of beer.
So first, what’s being measured in a hydrometer? Specific gravity (SG).
Specific gravity is the density of a liquid (in this case beer or wort) in comparison to water. It’s basically a ratio. Water has a specific gravity of 1.00, because the ratio of water to itself is 1.00!
Original specific gravity (OSG or OG) is the specific gravity of the wort. Wort is more dense than water, mostly because of the sugars, so effectively, taking the original gravity of a beer is seeing how much sugar is in the wort. It’s the potential of how much alcohol will be in the beer.
Final specific gravity (FSG or FG) is the specific gravity of the beer (usually) once fermentation is complete. It’s also useful to see how fermentation is going. Since sugars have been eaten in fermentation and turned into alcohol (which is less dense than water), the specific gravity will have gone down. Because of non-fermentable sugars and other things in beer, the final gravity usually won’t go below 1.00.
By comparing the original gravity to the final gravity, it’s easy to find out how much alcohol is in a beer. This is because it’s know that the only thing that came out was sugar, and the only thing that went in was alcohol. Knowing these things leads to a hand formula!
Calculating ABV through specific gravity
The ABV calculator above uses a slightly more complicated version of the more common ABV formula:
ABV = (OG – FG) * 131.25
While the above is pretty good for calculating the alcohol of beer, it’s a little bit inaccurate, especially as a beer gets stronger. Because of this, we’re using a more accurate formula, however it’s more complicated. The more complicated ABV formula is:
ABV = [76.08 * (OG – FG)/(1.775 – OG)] * (FG/0.794)
Obviously not as easy to jot down on a piece of paper while trying to brew, but that’s what the ABV calculator is for!
Converting Specific Gravity to Plato
The Plato scale measures the percentage of sugar by weight in a liquid. So, similar to specific gravity, getting the initial ⁰P and the final ⁰Plato can be used to calculate ABV.
However, since the alcohol by volume for the beer is already known, calculating the initial and final ⁰P is only used to know the percentage sugar in the beer. In this way, 1 ⁰P is the same as saying the beer is 1% sugar by weight.
The formula to convert specific gravity to Plato is:
⁰P = (668.72 * SG) – (205.35 * SG2) – 463.37
Where SG is either the initial or the final specific gravity.
Calculating Real Extract
Real extract is measured in ⁰P, so once again it’s the percentage of sugar in a liquid. Real extract measures how much percent weight by sugar was lost during fermentation.
Real extract is calculated as:
RE = (0.1808 * ⁰Pi) + (0.8192 * ⁰Pf)
Why isn’t it just final minus initial? Well, it’s measuring the percentage weight lost due to the conversion. The conversion is basically sugar to alcohol and carbon dioxide, so quite a bit stays there.
Attenuation calculates the percentage of sugar that was converted to alcohol during fermentation. The ABV calculator above gives two types of attenuation, apparent attenuation and real attenuation.
Apparent attenuation is approximate and simply used the initial and final Plato values.
AA = 1 – (⁰Pf/⁰Pi)
Using the real extract value to calculate real attenuation is more accurate as it’s using how much sugar was actually lost. The real attenuation is calculated as:
RA = 1 – (RE/⁰Pi)
Calculating Alcohol by Weight (ABW)
If you’re interested, all the calculated ABV and the final gravity can also be used to figure out the alcohol by weight of a beer. Most people use alcohol by volume, but some areas go by alcohol by weight. The formula to convert ABV to ABW is:
ABW = ABV * 0.78924
Calculating Calories in Beer
Some people care about their waistlines, so they want to know how many calories are in their home brew. The below formula is basically three in one: the calories in the alcohol of the beer, the calories in the sugars of the beer, then multiplied by 3.3 to turn it into a standard 330 mL bottle. If you’re using different sized bottles, change the 3.3 to the size of your bottle. For example, 500 mL is 5.0 instead of 3.3.
Cals = [(6.9 * ABW) + 4 * (RE – 0.1)] * FG * 3.3
And that’s how alcohol by volume is calculated and all the other numbers around calculating ABV.
If you’ve enjoyed this summary on calculating a beer’s alcohol content, learn how to calculate home brew IBU here. Or, if you’re about to make some home brew and are about to mash some grain, learn how to calculate strike water temperature here.