We’ve discussed Overproof rum before but after a recent chat with my Bajan brother-in-law, we felt compelled to take it one step further.

“Why do I see these bottles with 102% this and 151% that? How can you get 151% anything!?”

How indeed…well now for the magic! You can’t. Sorry Jerome but what you are in fact seeing on the labels of these wonderful spirits is a classic battle between old and new, imperial and metric, Proof and ABV.

So, let’s start at the end; what is ABV?

ABV is the acronym for Alcohol By Volume i.e. the number of millilitres of pure ethanol present in 100ml solution. The number of millilitres of pure ethanol is calculated by taking the mass of the ethanol and dividing it by its density (0.78924g/ml @ 20^{o}C). ABV is now the worldwide standard for labelling the content of alcohol in a spirit.

Back in 16^{th} century England, spirits were taxed at variable rates dependent on their alcohol content. It was discovered that if you soaked gunpowder in a spirit and the gunpowder still burned, the spirit was considered above proof (yay) and therefore taxed at a higher rate (boo). For rum that level of alcohol was 57.15% ABV and this became known as 100 degrees Proof.

The gunpowder test was replaced in 1816 with the specific gravity test which lasted until the 1^{st} January 1980. The test was to measure spirit with a gravity ^{12}/_{13} of water (923kg/m^{3}) and equivalent to 57.15% ABV.

It’s all about the math:

-> 57.15% ≈ ^{4}/_{7 }= 0.5714

-> 100^{o} Proof = 57.15% ABV ≈ ^{4}/_{7} ABV

-> 100% ABV = 100^{o} Proof (^{7}/_{4}) = 175^{o} Proof

e.g. 42.8% ABV = 42.8(^{7}/_{4}) = 42.8(1.75) = 75^{o} Proof

Just to make matters more confusing, in the US (c.1848) the proof system established was based on % alcohol rather than specific gravity whereby 50% alcohol was the equivalent to 100^{o} Proof.

So in summary:

UK: 57.15% ABV = 100^{o} Proof (1.75ABV = Proof)

US: 50% ABV = 100^{o} Proof (2ABV = Proof)

This would explain why Bacardi 151^{o} Proof is by virtue of US standards 75.5% ABV.

Hopefully that explains a little bit more about the confusing world of Overproof rum. All that’s left now is to enjoy them!