Reference

Proof vs ABV: the conversion and where it came from

2 minute read · The Blending School

In the United States, proof is simply alcohol by volume doubled: 100 proof is 50 percent alcohol. Everywhere else the bottle says ABV. The history of why is odd, and the math of converting is trivial.

The conversion

90 proof is 45 percent. 125 proof is 62.5 percent. 80 proof, the legal minimum for bourbon, is 40 percent. That is the whole conversion for anything bottled in the United States.

Where "proof" came from

The word is older than the arithmetic. In 16th century England, spirits were taxed by strength, and the test was to soak gunpowder in the liquor and light it. If it still burned, the spirit was "proved" strong enough for the higher rate; roughly 57 percent alcohol. British proof was later fixed by specific gravity, with 100 proof at about 57.1 percent ABV, which is why old British bottlings read as odd numbers today. The United States simplified it in the 19th century to exactly twice the ABV, and Britain switched to plain ABV in 1980.

The rest of the world

Scotch, Irish, Japanese, and nearly all other whiskies label ABV only. When blending those with American whiskey, convert everything to one unit first. It does not matter which, only that you do not mix them; a "45" that is ABV blended with a "90" that is proof is the same strength, and treating them as different is the most common arithmetic error in home blending.

Which unit to blend in

For American whiskey, proof, because the labels say proof and the numbers are bigger and easier to hear the difference in. For a bottle that mixes American and world whiskies, convert the world whiskies to proof once, write it on a sticker, and never think about it again. Thief & Blend works in proof throughout, and the proof to ABV tool handles the conversion when you need it.