Bourbon whiskey may leave the still at no more than 160 proof in the United States.
This limit equals 80% alcohol by volume. Distillers commonly reduce the spirit with water before placing it into new, charred oak barrels, because bourbon must enter its barrels at no more than 125 proof, or 62.5% ABV.
The distillation limit helps preserve flavor compounds from the fermented grain mash. Distilling to a very high strength can produce a more neutral spirit, while a lower proof retains more of the character created during fermentation and distillation.
The 160-proof rule is different from bourbon’s other legal requirements. Bourbon must use a grain mixture containing at least 51% corn, mature in new charred oak containers, and be bottled at a minimum of 80 proof. The 160-proof figure concerns the spirit immediately after distillation, not the strength sold in shops.