Under U.S. law, wineries must state the exact percentage of alcohol the wine contains on the wine’s label.
While it’s true that wineries must state an amount of alcohol, there’s a lot of wiggle room in the percentage stated. In the U.S., wines containing 14% or less alcohol, can be labeled plus or minus 1.5 percentage points. So that “light, elegant” wine you bought which was labeled 12.5 percent alcohol could actually contain 14%. For wines over 14% alcohol, a 1% variance is allowed. So, a wine labeled 14.5% alcohol, could contain 15.5%. No wine (except for fortified wines) is ever over about 16.5% alcohol because at that concentration, yeasts die from the very alcohol they are making, preventing them from making more.