Converting proof to percentage is helpful if you want to know how much alcohol is in a tumbler of vodka or a glass of wine. The term stretches back to the 18th century, when workers proofed liquor at a distillery by adding some gunpowder to it and then setting it alight. Liquor that didn't react was deemed too weak, and liquor that burnt yellow was too strong, and liquor that burnt blue was just the right proof. You can perform a simple calculation to determine the alcoholic content of your beverage based on its proof.
Write down the proof of your alcoholic beverage.
Divide the number by two.
Write down the answer, which is the alcoholic percentage of your beverage.