Drink Life Beverages

Understanding British Imperial Proof Strength

I see a lot of questions about the old British “imperial proof” system

In the American definition, the “proof” is twice the alcoholic strength by volume

This particular bottle was old enough that it used the imperial proof system

The imperial proof system is one of those things that seems super complicated until you understand it,

To do that, let’s momentarily look at something completely unrelated yet quite instructive

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

A quick look at Wikipedia shows that the moon’s gravity is 16

How much more? Again, Wikipedia has the answer: 265% more than the earth, so 2

The important point here is that we’ve chosen a unit of measurement—the gravity of the

With this in mind, we’re now better prepared to understand the imperial proof system

Now, imagine that some global authority decides that their favorite spirit is the standard by which all

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

With one spirit, our magic measuring device says that it’s 10% more than the reference spirit

The parallels between units of gravity and units of “reference strength” are very simple to grasp

Without going deep into the history of British excise law, the British government didn’t use ABV

Terms like “underproof” and “overproof” were percentages and relative to proof strength

The “151” refers to its strength in the American proof system

Thus, “151 rum” would be 132 imperial proof, or “32 percent overproof,” if you prefer

” The closest thing is a “proof gallon,” defined as a spirit with 50% ethanol

A lot of brands play fast and loose with their terminology, with many describing anything 50% ABV or

” Since the Americans haven’t defined overproof, while the Brits did, I’m sticking with the British

However, if you come across an old bottle use the system, you now have the understanding of