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What Does "Proof" Mean in Alcohol?

Ever wondered what “proof” in alcohol means, or how it relates to alcohol strength. If so, read on and prepare to be unmystified.


“Proof” indicated the alcoholic strength of a drink in certain countries such as the USA. Numerically-speaking, proof is defined as twice the alcohol content by volume (ABV). For example, a whisky with 50% alcohol (50% ABV) is a 100-proof whiskey; and a 140-proof rum would contain 60% alcohol (60% ABV).


The world “proof” itself came about in the 18th century when sailors, unloading their ships of cargo and liquor, had only one way to tell the strength of the liquor they were carrying – by mixing a bit of the spirit with a pinch of gunpowder and dropping a lit match into the mixture. If it ignited, it was proof that the liquid was at least half alcohol. If it remained unlit, it was supposed that it had a lower alcohol content. As you might expect, this method wasn’t entirely accurate. One such reason was that the flammability of the liquor was dependent on its temperature, which was not kept consistent. 

In the US, a baseline was settled upon in the mid-19th century that made a 50% ABV spirit exactly “100-proof”. This is still the measurable method today, meaning that in order for a spirit to be “proof”, it must be 50% ABV.


“Proof” is certainly an important term to know the definition of when purchasing hard liquor. Here are a few more that might be of use.

Cask Strength – this simply means that the spirit has been bottled at the strength it was in the cask, with no water added to dilute it. This term is usually used for whisk(e)y and rum, although some tequila also labels its wares as such (or as “Still Strength”, which means the liquid has been bottled at the strength it came off the still)

Barrel Proof / Barrel Strength – the same as Cask Strength, meaning the proof is the same as it was whilst in the barrel

Navy Strength – usually used in reference to gin or rum, this indicates a stronger spirit (traditionally around 57% ABV /114-proof, though it can go even higher)

Overproof – this term is interchangeable with Navy Strength, indicating a spirit over 57% ABV / 114-proof

Single Cask – not related to strength, nevertheless an important one to know. This means that the bottled spirit was produced from one cask, instead of being a blend of multiple casks. Individual casks can affect the nature of ageing spirits considerably, depending on a number of factors such as wood type, age, size and charr. Single cask bottlings tend to be from casks that have imparted characterful flavours to the liquid in them

Double Barrel / Double Wood – like Single Cask, this isn’t about strength. It refers to the number of times that a spirit has been aged in wood barrels. This is not required in labelling but some producers choose to add this information to their bottles to denote complexity of flavour

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