I see a lot of questions about the old British “imperial proof” system. It confuses a lot of folks, which is understandable. Here’s an example: someone recently posted a picture of an old bottle. The label says “75% proof” as well as “43% ABV.”
In the American definition, the “proof” is twice the alcoholic strength by volume. In the case of the above bottle, twice 43% ABV is 86 proof.
Yet the label ways 75% proof. What gives?
This particular bottle was old enough that it used the imperial proof system. Let’s take a look at that system.
The Gravity of the Situation
The imperial proof system is one of those things that seems super complicated until you understand it, then it’s incredibly easy.
Rather than jump straight into proof math, let’s understand it conceptually. To do that, let’s momentarily look at something completely unrelated yet quite instructive.
Let’s talk about gravity. Scientists have technical terms and units to describe it, but we’re going to keep it simple. The earth we all live on has a gravitation pull downward. The gravitational pull is essentially the same anywhere we go on the earth’s surface. Let’s give that amount of gravity a name: “standard gravity.”
Now, if we teleport to the moon, we know the gravitational pull is less. But how can we describe how much less? Perhaps we might describe it as a percentage of the Earth’s standard gravity. A quick look at Wikipedia shows that the moon’s gravity is 16.5 percent of the Earth’s standard gravity.
Next, let’s zoom off to Jupiter, where we’ll weigh much more because Jupiter has a higher gravity than earth. How much more? Again, Wikipedia has the answer: 265% more than the earth, so 2.65 Earth standard gravities.
The important point here is that we’ve chosen a unit of measurement—the gravity of the earth—and we’ve expressed the gravity of other celestial bodies in units of that measurement.
With this in mind, we’re now better prepared to understand the imperial proof system.
What Your Proof?
Clear your mind of anything you know about the alcoholic strength of spirits. Forget about ABV; forget about “proof.”
Now, imagine that some global authority decides that their favorite spirit is the standard by which all other spirits should be measured. We shall call it the “reference spirit.” We have no idea what its alcoholic strength is, so we can only say it’s of “reference strength” — whatever that means.
Next, think about how we might describe the strength of another spirit. Luckily, we have a magic measuring device that can tell us the strength of any spirit relative to this reference strength.
With one spirit, our magic measuring device says that it’s 10% more than the reference spirit. We could say that spirit is “110% reference strength.”
For another spirit, the device declares it to be 70% of the reference strength. We could put “70% reference strength” on the label.
The parallels between units of gravity and units of “reference strength” are very simple to grasp.
As it just so happens, centuries ago, the British government actually declared a “reference strength.”
They called it “Proof strength.”
Understanding Proof Math
Without going deep into the history of British excise law, the British government didn’t use ABV way back in the day. Instead, they defined what “proof” meant. In modern terms, it was equivalent to 57.14% ABV.
Terms like “underproof” and “overproof” were percentages and relative to proof strength.
Let’s look at some examples:
10% overproof
110% of 57.14% ABV = 62.8% ABV.
75% Proof
75% of 57.14% ABV = 42.9% ABV
Navy Strength
Navy strength was defined as “4.5 percent (“degrees”) under proof”
100% – 4.5% = 95.5%
95.5% of 57.14% ABV = 54.5% ABV.
Yes, navy strength is not 57% ABV. Many brands have this wrong. Nor is it overproof. Technically, it was underproof.
Doing it the other way – 151 Rum
“151” strength rum is popular in cocktails. The “151” refers to its strength in the American proof system.
It’s also a great example for doing the math backward to convert from American proof or ABV to imperial proof.
151 American proof = 75.5% ABV.
75.5 is 32% more than 57.14: 75.5 / 57.14 = 1.32. Or 132%, if you prefer.
Thus, “151 rum” would be 132 imperial proof, or “32 percent overproof,” if you prefer.
Let’s Talk About “Overproof”
In the imperial proof system, what “overproof” means is very clear. It’s anything stronger than 57.14 ABV.
The American system does not define “overproof.” The closest thing is a “proof gallon,” defined as a spirit with 50% ethanol. They’re not the same thing, however.
A lot of brands play fast and loose with their terminology, with many describing anything 50% ABV or higher as “overproof.” Since the Americans haven’t defined overproof, while the Brits did, I’m sticking with the British definition in my writing.
Summary
Mercifully, the British Imperial proof system is no longer in active use today. However, if you come across an old bottle use the system, you now have the understanding of how to interpret what it means.
As always, I enjoy reading your posts. Thanks for the edification and for taking your readers on a journey.
A shame that the actual Imperial reason for the obscure ABV isn’t mentioned in the article.
We use proof to reference firearms now. But back in the day, all alcohol stowed aboard warships had to be overproof. ie if it came into contact with their gunpowder it could still be used as a propellant.
> A shame that the actual Imperial reason for the obscure ABV isn’t mentioned in the article….But back in the day, all alcohol stowed aboard warships had to be overproof.
It’s a nice, oft-told story but does not seem to be backed up by any historical document I’ve seen, and I have hundreds on this topic.
As a point of discussion, the 1939 Victualling manual states: “The average strength on delivery is 40° over proof, and the issuing strength is 4·5 under proof. This latter strength was fixed on the 19th February, 1866.”
If overproof rum was required so that gunpowder would ignite, why would the navy issue rum to ship at less than that strength?
Furthermore, the same victualling manual continues: “The term “proof” takes its origin from the following method formerly employed by the Customs. Gunpowder was moistened with the spirit to be tested, and the alcohol ignited; if it fired the powder, it was said to be proof, or over proof, but if the spirit burnt off and left a soddened mass it was considered to be under proof.”
The use by customs agents is mentioned, but not by navy pursers. Nor is there any mention of overproof requirement to ensure a ship’s gunpowder will ignite.
The above is also in line with Pack’s Nelson’s Blood book, which states: “In fact, it was impossible to establish the proof (or strength) of naval rum accurately until 1816, at which time the Sikes’ hydrometer was invented. For many years prior to this, the ship’s purser or ‘pusser’ as he was called was responsible for testing and issuing rum at proof by a rough rule of thumb method said to have been invented at the Royal Arsenal. Pure rum was mixed with a little water to which was added a few black gunpowder grains, so that when the sun heated the mixture through a burning glass, the gunpowder just ignited but did no more. Legend has it that in carrying out this test, the explosion of too strong a mixture would blow a purser sky-high allowing everyone to help himself! Too weak a mixture failed to ignite, and the purser could be punished for watering the rum.”
Again, the gunpowder test is cited as a means of measuring strength, but no mention is made of that strength being required so that a ship’s gunpowder will ignite.
If you can find historic text that supports your claim, I’d very much like to review it
Thanks!
Matt