Spirit strength may be designated in several waysâweight per gallon, percentage by weight, or percentage by volume, all these having reference to absolute (i.e., pure) alcohol and water. There are other standards in common useâe.g., U.S. proof spirit, which is 50 percent alcohol by volume. Each degree of U.S. proof represents 0.5 percent alcohol, so that a liquor having 50 percent alcohol is termed 100 proof. British proof is based on a specific concentration of alcohol, a 50 percent alcoholic content being equivalent to 114.12 U.S. proof. British proof is expressed as degrees over or under proof (that is, over or under 50 percent alcohol), while U.S. proof is expressed in direct proof figures. The metric Gay-Lussac system simply states the percentage by volume of alcohol in a distilled liquor.