Whisky Names Explained: 100 Proof, the British vs. American system

Monday, 10 August 2026 at 14:21

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Anyone holding a bottle of Signatory Vintage 100 Proof might assume it’s a whisky at 50 percent alcohol by volume. Not so. In the Signatory Vintage series, every whisky is bottled at 57.1 percent ABV. The reason lies in the difference between the British and American proof systems. You can read how that works in this Whisky Names Explained.

Let’s start at the beginning: what does the word proof actually mean? In English, proof means evidence. When we’re talking about spirits like whisky, it essentially means proof of strength. Back before modern equipment existed to measure alcohol, a distillery could “prove” the alcohol content another way.

The British proof system

In the United Kingdom, proving alcohol took a rather unusual form. The spirit was mixed with gunpowder and then ignited. If the liquid wouldn’t burn, it was “under proof.” If the gunpowder blend did catch fire, the spirit was “proof.”

In 1816, Bartholomew Sikes invented the hydrometer, making it much easier to determine a whisky’s alcohol content. The proof threshold established by the gunpowder test later averaged out at 57.1 percent alcohol by volume (ABV). By then, the proof system was firmly ingrained, so 57.1 percent ABV became 100 proof. The United Kingdom didn’t switch to expressing alcohol content in ABV until 1980.

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The American proof system

Now that we know the British proof system was based on a gunpowder test, let’s look at the American system. In the United States, “US proof” was adopted as a way to express a spirit’s strength. ABV had been the international standard for years, but Americans like to do things their own way. US proof began appearing alongside ABV more and more on bottles of bourbon and American whiskey.

Much like the Bottled in Bond tradition, read more in our article on proof - US proof gradually became a mark of prestige for American whiskey and bourbon, often at higher strengths. US proof isn’t determined by an old-fashioned gunpowder test. It’s simply a bit of math: take the ABV and multiply by two. A whisky at 46 percent becomes 92 proof, and 100 proof corresponds to 50 percent ABV.

FAQ

What does 100 proof mean in whisky?
That depends on the proof system used. In the old British system, 100 proof equals 57.1% ABV, while in the American system, 100 proof equals 50% ABV.
Why is Signatory Vintage 100 Proof 57.1%
ABV? Signatory Vintage uses 100 proof in reference to the historical British proof system, in which 57.1% alcohol by volume was considered 100 proof.
How do you convert US proof to ABV?
In the American system, proof is simply twice the alcohol percentage. So a whisky at 46% ABV is 92 proof, and a whisky at 50% ABV is 100 proof.