"Proof" in the US means the number ie 80 proof divided by 2 is the alcohol %. So 3 ounces of 80 proof vodka is equal to 3 x 0.4 = 1.2 ounces of pure alcohol or pure vodka. Proof came from the way the whiskey in the US was tested. It was mixed with gunpowder and if it ignited then it was 100% ie had no water added.
