Alcohol formula

Why does the proof double the alcohol percentage?

While browsing the shelves of your local liquor store, you will likely come across one of two methods used to measure the alcohol content of a bottle. The first is Alcohol by Volume, or ABV, which is probably the measurement you’re most familiar with – it explicitly tells you the percentage of alcohol present in the liquid. The second method of measurement is evidence, which may be less well understood than its counterpart, especially considering how much it varies from country to country.

In the United States, the measure of proof operates on a scale of zero to 200, and an alcohol’s proof is exactly twice its alcohol percentage by volume. For example, if a vodka is labeled as 90 degrees, it contains 45% alcohol by volume.

The method of proof labeling in the United States dates back to 1848, when the US government chose 50% alcohol by volume as its benchmark, with 100 being used as the corresponding proof. For a liqueur to be considered “proof alcohol”, it must be 100 degrees, with an ABV of 50% or more.

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American drinkers might be shocked if they recently bought a spirit in France, where they will find that the 45-degree alcohol is much stronger than they expect back home. Indeed, in France, proof and spirit exist in a ratio of 1 to 1, with 100% ABV corresponding to 100 proofs.

The term proof refers to a practice the British government began in the 1500s to determine the amount of alcohol present in a given spirit. At the time, England had an “evidence tax” in which spirits with high alcohol content would be taxed at a higher rate. In order to determine this level of alcohol, the government would soak a firearm pellet in alcohol, then ignite the gunpowder; if the alcohol content was high enough for the gunpowder to ignite, the alcohol was considered a “spirit of proof”. Basically, the test was a method of proving high alcohol content in liqueurs, hence the term, proof.

In 1816, the British government abolished this method and introduced the Customs and Excise Act as a more scientific method of determining proof. The law states that for a liquor to be considered 100 degrees, its weight must be equal to twelve-thirteenths of that of water of equal volume. This method was later standardized in 1952, with the ratio of proof to ABV set at 4 to 7, meaning that a 100 proof liquor is about 57% ABV. Luckily for Brits who aren’t looking to take a math test to figure out how much alcohol is in their vodka, the country started adopting the simple ABV system in 1980.

For now, as long as you stay in the US while exploring your next liquor store, you can count on proof that it’s exactly double ABV.