Why is proof two times the percentage of alcohol?

Answer 1

Americans use proof, and other parts use percentages. so say 80 proof is equivalent to 40%. 80 proof doesn't mean it has more alcohol it's just the different way of saying how much alcohol in a bottle per volumn.

Answer 2

I've wondered that for a while. The only thing I can come up with is that it has a larger number, so some people might think it is stronger, but I hope that isn't the reason.

Answer 3

Further complicating discussions of amounts of alcohol is the difference between potency measures based upon "percent of alcohol by volume" and measures based upon "percent of alcohol by weight." Since alcohol weighs less than water, any discussion about the proportion of alcohol in a beverage must specify whether the comparison is "by weight" or "by volume." A pint of water weights about one pound (16 ounces). A pint of pure alcohol weighs about 12.8 ounces. If one pint of pure alcohol is mixed with one pint of water, the result is a beverage that is 50% alcohol "by volume." If one pound of pure alcohol is mixed with one pound of pure water, the result is a beverage that is 50% alcohol "by weight." Since one pound of water 16 fluid ounces, and one pound of alcohol is about 20 fluid ounces, the 36 total fluid ounces in a resulting beverage is about 55.5% alcohol "by volume," although it is 50% "by weight." As an example of the importance of specifying the system used to measure the percentage of alcohol in a beverage, Coors Brewing Company promotional materials describe "Zima" as containing 3.7% alcohol by weight; 4.6% alcohol by volume. To convert measures by weight into measures by volume (or visa versa), use a conversion factor of 0.79.
Measures of Potency; "Proof"

The strength of an alcoholic beverage is usually expressed as either the percentage of absolute alcohol by volume, or as "proof." In the USA, proof is a measure of the amount of absolute alcohol in distilled spirits, and is calculated as the number of parts of alcohol in 200 parts of beverage at 60 degrees Fahrenheit (a beverage that is 50% alcohol is "100 proof;" a beverage that is 43% alcohol is "86 proof"). Under the American system, one "proof" is equal to one-half percent of alcohol by volume.

The concept of "proof" emerged prior to the 18th century, when few people had the equipment necessary to make precise measurements of the percentage of alcohol in a beverage. Exotic formulas, such as igniting a mixture of equal parts of the beverage and gunpowder, were used to test the proof of distilled spirits.

A different system for measuring proof is used in England, In England, proof is measured by comparing equal volumes of water and the beverage at 51 degrees Fahrenheit. 100 proof is set at the point at which the beverage weighs 12/13th as much as water. This means that a beverage that is said to be 100 proof under the British system is 114.2 proof under the American system. A beverage that is 100 proof under the American system is said to be 87.6 proof under the British system.