A unit is 10 ml of pure alcohol. Vodkas sold in the US are most commonly 40 percent alcohol by volume. A 750 ml bottle of vodka would contain 30 units of alcohol.
Federal law mandates that containers of beverage alcohol must be accurately labelled with the percentage alcohol by volume, or ABV, and the total volume of the container. Since a unit of alcohol is defined as 10 ml of pure alcohol, the calculation is a matter of multiplying the ABV by the container volume and then dividing the total by 10.
Alcohol content is also often expressed in terms of proof. Proof is double the percentage alcohol by volume. Therefore, eighty proof vodka is 40 percent ABV.