1.2 Alcohol proof and alcohol volume
In the US, the alcohol content is measured by ‘alcohol proof’ which is twice the percentage of alcohol by volume (ABV), used in Europe.
So, in the US, 'eighty proof' spirit is actually only 40% alcohol. This isn’t the alcohol producers trying to over-inflate their numbers – there is a historical reason why we have measurements of both alcohol proof and alcohol volume, both of which relate to the actual content of alcohol in a drink.
It began when sailors in the British Navy came up with the idea of mixing alcohol and gunpowder. When the sailors did this, they noticed that gunpowder in alcohol would ignite – but only when the alcohol was not too watered down. Sailors would ‘prove’ the rum acceptable by pouring some onto gunpowder and lighting it. If it burned steadily with a blue flame, it was considered ‘100 proof’. If it did not burn, it was ‘underproof’. And if it burned too quickly it was ‘overproof’. The flame was considered the 'proof' of the alcohol content.
Historically, sailors weren't the only people interested in determining the alcohol content of various drinks. Spirits were taxed according to their alcohol content. Tax collectors used hydrometers, similar to those you saw in Week 1, to measure alcohol content.