Alcoholic strength

Beer ranges from less than 3% alcohol by volume (abv) to almost 30% abv. The alcohol content of beer varies by local practice or beer style. The pale lagers that most consumers are familiar with fall in the range of 4–6%, with a typical abv of 5%. The customary strength of British ales is quite low, with many session beers being around 4% abv. Some beers, such as tafelbier (table beer) are of such low alcohol content (1%~4%) that they are served instead of soft drinks in some schools.
The alcohol in beer comes primarily from the metabolism of sugars that are produced during fermentation. The quantity of fermentable sugars in the wort and the variety of yeast used to ferment the wort are the primary factors that determine the amount of alcohol in the final beer. Additional fermentable sugars are sometimes added to increase alcohol content, and enzymes are often added to the wort for certain styles of beer (primarily 'light' beers) to convert more complex carbohydrates (starches) to fermentable sugars. Alcohol is a waste product of yeast metabolism and is toxic to the yeast; typical brewing yeast cannot survive at alcohol concentrations above 12% by volume. Low temperatures and too little fermentation time decrease the effectiveness of yeasts, and consequently decrease the alcohol content.

Exceptional strength beers
Some brewers have used champagne yeasts to increase the alcohol content of their beers. Samuel Adams reached 20% abv with Millennium and then surpassed that amount to 25.6% abv with Utopias. The strongest beer sold in Britain was Delaware's Dogfish Head's World Wide Stout, a 21% abv stout which was available from UK Safeways in 2003. In Japan in 2005, the Hakusekikan Beer Restaurant sold an eisbock, strengthened through "freeze distillation", believed to be 28% abv. The beer that is considered to be the strongest yet made is Hair of the Dog's Dave — a 29% abv barley wine made in 1994