A 750-milliliter bottle of 86 proof whiskey contains 43 percent alcohol by volume. A 12-ounce can of beer with 5 percent alcohol is 10 proof. The alcohol content can be found on the front or back labels of virtually every alcoholic beverage. The proof is determined by measuring the amount of ethanol -- the only consumable form of alcohol -- in a liquid held at a temperature of 60 degrees Fahrenheit and multiplying this number by two. The temperature is a factor because 100 percent alcohol will begin to evaporate above a certain temperature.
Farmers Discover Distillation
The term proof as it relates to alcohol content refers to the 18th century method used as "proof" that there was 57 percent ethanol in whiskey. Although the manufacture of alcohol for consumption dates back more than 1,000 years, the tools to precisely measure alcohol in liquor, wine and beer, did not exist until relatively recently. This became a problem in The United States when local farmers began producing whiskey for commercial distribution as a lucrative alternative to selling corn as grain.
The Proof Is in the Pop
To maximize profits, some producers began to water down their whiskey so that it contained less than the expected 57 percent ethanol. In an effort to verify that a manufacturer's whiskey was potent enough to meet this requirement, distributors would add gunpowder to a sample of whiskey and light it. If the whiskey contained 57 percent ethanol, the gunpowder would react with the whiskey and burn steady and blue like a lit match. If it was "overproof," or more than 57 percent ethanol, the flame would ignite and burn very quickly. If it was under 57 percent, no reaction would occur. The dramatic and immediate reaction was sufficient "proof" for most consumers at the time who wanted to verify whether the whiskey met the standard.
Differences in Proof Measurement
The methods for measuring the alcohol content in beverages differs internationally. Most countries follow the International Organization of Legal Metrology's standard of alcohol measurement. The process involves distilling the alcohol to isolate it. The alcohol is then weighed and expressed in percentage by volume. The French method is expressed in degrees Gay-Lussac and is equivalent to the alcohol-by-volume measurements used in the United States.
It All Adds Up
Despite the differences in proof or alcohol-by-volume measurements in wine, liquor or beer, one standard rule of thumb exists for any of these alcohols -- 5 ounces of red or white wine, 12 ounces of beer and 1 1/2 ounces of liquor all contain the same amount of alcohol. So when it comes to measuring alcohol content, the type of drink is important in determining its proof.