Proof is one of those terms most of us use to describe our whiskey without entirely understanding what it means. Legally, a modern-day proof signifies twice the percentage alcohol by volume in the United States. You can figure this out because the alcohol by volume percentage is required to be on every bottle — usually right next to the proof. In other words, it's entirely redundant. So why is it really there?
We asked whiskey historian and author of The Rise, Fall, and Rebirth of American Whiskey, Fred Minnick, to help us make sense of it all. Minnick, of course, blames the British.
If you go back 300 years, before instruments allowed you to determine the density of a liquid, a consumer would determine the alcohol content of a liquid by pouring it over gunpowder, lighting the powder on fire, and watching what happened. “If the alcohol was diluted with water or other non-flammable liquid, the alcohol flame went out without sparking the wet gunpowder,” says Minnick. And if it was not diluted, the gunpowder caught fire and eventually sparked. That was considered the “proof.”
So proof was essentially the point at which there was enough alcohol in a liquid for gunpowder to still spark. There could be more than the minimum requirement, in which case a spirit was considered “overproof.” Which is why, today, proof can be a number over 100.
As with many things, the terminology stuck even after the method became outdated.
“The hydrometer was invented in the 1730s, nullifying this crude “proof” method,” says Minnick, “but the name stuck.” And even when the original hydrometer was replaced by more advanced ones in the late 1700s that could measure percentage levels above proof, the name continued since, according to Minnick, it was still “a trusted symbol in consumer expectation.”
The U.S. adopted a simple measurement of alcohol by volume in the 1840s, and modern distillers are now required to print the percent of alcohol by volume, or percent ABV, on the bottle. But they are also allowed to print the proof, legally defined as twice the percentage of alcohol by volume, even though that’s largely redundant.
Today, the traditional term has itself become a simple bit of marketing. After all, 100 proof sounds a lot cooler than 50 percent ABV, doesn't it?