35 ILCS 505/1.18: "Proof gallon" means one gallon of 100 proof alcohol.

For example: One gallon of 200 proof alcohol is two proof gallons. For the purpose of this definition "proof" is a term used to describe the strength of an ethyl alcohol solution and is a number that is double that of the percent alcohol in the solution. For example: "100 proof" means 50 percent alcohol.