Whether it’s wine, beer, spirits, cider, or hard seltzer, every alcoholic beverage label must display its alcohol content. The numerous ways in which this can be written can be confusing.
The two primary methods of indicating the alcohol content of a beverage are alcohol by volume (ABV) and proof. In the United States, a spirit’s proof is simply double its ABV. This means that a bottle of 90-proof bourbon has an ABV of 45%, while a bottle of 151-proof rum has an ABV of 75.5%.
So, where does the term “proof” come from and why do we still use it?
Most sources point to 16th century England, where higher taxes were imposed on spirits with a certain strength. Since there were no tools available to measure the exact alcohol level of a spirit easily and accurately, its strength was tested using a much simpler method: Will it catch fire? If the liquid was strong enough to burn (or ignite a gunpowder pellet soaked in it), it was considered proof that the bottle was strong enough to warrant the extra tax.
A scale was created with the number 100 chosen as the “proof” at which a spirit would burn. Anything lower was exempt from the higher tax.
Of course, this is generally a poor way to measure the alcohol content of a spirit. Combustibility depends on factors beyond ABV, as anyone who has ever lit a 90-proof whiskey on fire can attest. The temperature of the liquid also plays a significant role. Warmer liquid and ambient room temperature allow more alcohol vapor to escape, increasing flammability. In the absence of a more scientific process, the “proof” of a liquid could vary between a warm day and a cold one.
However, this explains why drinks like wine and beer were historically never referred to by their proof. They would not ignite and were not subject to the extra tax, so there was nothing to prove.
By the 19th century, more accurate techniques had been discovered to measure the alcohol content of spirits, such as determining the liquid’s specific gravity or density. As the alcohol content increases, spirits generally become lighter. This is why different spirits can be floated on top of each other in cocktails. In England, a “proof” spirit was officially defined as one that had 12/13th the density of an equal volume of distilled water at 11°C (51°F).
In other words, this meant that a spirit with an ABV of about 57% was defined as a “100 proof” spirit in the United Kingdom.
If you find this math confusing, you’re not alone. Around 1848, the United States, seeking a simpler system, defined a spirit’s proof as simply double the alcohol percentage. Thus, in the U.S., 100 proof became synonymous with 50% ABV, which is coincidentally around the same alcohol level that allows a spirit to easily ignite.
Meanwhile, a French chemist named Joseph-Louis Gay-Lussac found the arbitrary assignment of numbers by politicians setting tax standards to be inefficient, and in 1824, he created his own system. On the French scale, measured in degrees, 100% alcohol was simply 100 proof, while plain water was 0 proof. In France, the proof was the ABV.
As a result, a bottle of liquor with an ABV of 50% could be designated as 100 proof in the U.S., 50 proof in France, and 87.6 proof in Britain.
Fast forward to modern times, Europe and the rest of the world have largely abandoned the convoluted proof system. European Union guidelines now require the alcohol-by-volume to be stated as a percentage. In the U.S., the double-the-alcohol proof system is still used on some bottles, but it’s mostly a matter of tradition, and only the ABV is required to be listed by law.