That depends on what you mean by "naturally".
There are some strains, such as the "Super High Gravity Ale Yeast" by WyLabs, which can handle up to 25%.
But there are techniques, such as freezing the beer to create a more concentrated product, which have been used to get up to 60+%.
In this case, alcohol is not added , but rather, water is removed , altering your ABV.
beer.stackexchange.com/users/27/object88
The reason there's a limit is once the alcohol content is too high, it kills the yeast, so fermentation stops and no more alcohol is produced.
I think it's generally around 15% alcohol by volume, but the exact amount will depend on the type of yeast.
EDIT:
The Wikipedia page on yeast in winemaking has some details on when different types of yeasts die out.
beer.stackexchange.com/users/68/hunse
I am taking this from a website cited at the bottom.
Popsci has a LOT of cool beer articles.
Fractional freezing is the best way to increase the alcohol content of beer.
Fractional freezing --
"jacking" in old parlance -- has a long history in the United States.
The beverage applejack was produced using this method by first fermenting apple juice into hard apple cider.
Then barrels of this cider were left outside during the winter and the connoisseur would occasionally fish out the frozen chunks of water, leaving an ever-concentrated batch of hard alcohol behind.
At some point in the 20-25 percent ABV range, the liquor would stop freezing at ambient temperatures and the booze was ready to consume as "Jersey Lightning.
" It was also used as currency.
Moving into the malt-beverage world, brewers in Germany use fractional freezing to make eisbock.
These brews are usually just regular bock beers, which clock in at 6 percent ABV, freeze-concentrated to something in the 13 percent ABV range.
Frankly, there are probably easier ways to get high-proof beers (make a barleywine, for example) than by starting with a mid-strength one and concentrating it, but there's a market for eisbocks and it's possible to find them in the US as well.
A more infamous set of freeze-concentrated beers was made by the Scottish brewery BrewDog in their quest to make "the strongest beer in the world.
" The first was Tactical Nuclear Penguin, a beer that started with an ABV in the teens and ended at 32 percent ABV.
Then a little brewing war started up between BrewDog and German brewery Schorschbräu to make ever-stronger beers.
Schorschbräu made a Schorschbock at 40 percent ABV, BrewDog countered with Sink the Bismarck at 41 percent ABV.
Then came another beer at 55 percent ABV (that's 110 proof, my friends).
Schorschbräu is current record-holder with Schorschbock 57, at 57 percent ABV.
Article
beer.stackexchange.com/users/22/BryceH
Note that Ann Hagen ("Anglo-Saxon Food and Drink") argued that the strongest naturally brewed drinks available in Anglo-Saxon times were probably strongly brewed meads
at least 20% abv.
Her basis is in discussions of specific gravity of the resulting beverages in historical records.
I think it is likely close to that and no more because I see other reasons to wonder if specific gravity could have been reduced by other additives (in particular henbane).
I love studying history.
It is useful in so many fields!
In general getting much above 10-15%abv requires specialty yeasts or concentration techniques, and getting much above 20% is going to require some sort of concentration.
I have however seen cases where these rules go out the window in wine making experiments (a friend of mine accidently brewed a mango wine with 22% abv using standard wine yeasts, which surprised us both).
beer.stackexchange.com/users/117/Chris Travers
There is a beer called "Snake Venom", by "Brewmeister" a scottish brewery.
The alcohol content is 67.5% .
It has been verified by Trading Standards and is officially named the world´s strongest beer.
The wiki entry states that they use the process of fractional freezing to achieve such a high percentage of alcohol.
beer.stackexchange.com/users/4175/bweber
28% ABV is the best I have managed with my yeast though it needs plenty of sugar added to whatever you are brewing apart from mead.
Getting the yeast from the boot's from
it's original 8% to 20% took 8 years getting it from 20% to 28%
has taken 25 year's
and I haven't been able to get any increase on that 28% in the last 5 years though it is getting more consistent & given enough sugar
it rarely dips below 28%.
Ignore any references to Brewmeister here
there "cold brewing" process is Fractional freezing more conmanly called Freeze distillation where they remove water by freezing it out this has nothing to do with the brewing process.
beer.stackexchange.com/users/7052/Bovski
I just finished a batch of Applejack.
After freeze distillation, I obtained a 19% ABV, however the yeast kicked back on STRONGLY after it warmed up again.
I had to boil it to kill the fermentation.
Different yeasts can handle different ABV before being killed off by the alcohol.
I used Lavlin EC-118.
For your yeast it may differ.
beer.stackexchange.com/users/8234/CipherBytes
What is the highest level of alcohol you can achieve when brewing beer naturally, without adding alcohol?  I know that many strong, popular beers (~10% alcohol) are simply mixed with alcohol, but what level of alcohol can be achieved in natural brewing process?
What is the highest alcohol content achievable through brewing?
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<brewing>
beer.stackexchange.com/questions/110
beer.stackexchange.com/users/21/Danubian Sailor
S6: In general  getting much above 10-15%abv requires specialty yeasts or concentration techniques, and getting much above 20% is going to require some sort of concentration.  The reason there's a limit is once the alcohol content is too high, it kills the yeast, so fermentation stops and no more alcohol is produced. But there are techniques, such as freezing the beer to create a more concentrated product, which have been used to get up to 60+%.   While there are some exceptions to this rule, and some yeasts that can handle higher levels of fermentation, the limit is usually about 20% with normal fermentation techniques. Fractional freezing is an effective way to concentrate a beer and thereby increase its alcohl content.
While there are some exceptions to this rule, and some yeasts that can handle higher levels of fermentation, the limit is usually about 20% with normal fermentation techniques. Fractional freezing is an effective way to concentrate a beer and increase its alcohl content.