Updated: Jun 27
What is proof and why is it different than alcohol by volume (ABV) that you see on your bottles? Well, ABV is pretty straight forward, what percent of the liquid in the bottle is alcohol? This is a very universal measurement. No matter where on the globe you are, Vodka is still 60% water anyway right? (Think about that). So ABV seems to make sense, what then is Proof? Proof predates ABV by far and to understand it we will need a bit of a history lesson.
There are many arguments about the exact origins of distilled spirits with some claims placing the beginning 2000BC in Mesopotamia, others that China laid claim to the first crude distilled spirits. There are stories of water distillation in Greece as well, around 100AD, but not of alcohol. While there is conjecture around the true origins, there begins to be a trail of possible refinement found in Alchemy writings that discuss dates as early as the 400AD. These writing begin to discuss the use of an early three arm pot still known as a Tribikos that was being used by Mary the Jewess, a highly revered alchemist and rumored to be a teacher of the Greek philosopher Democritus. Alchemy always to use chemical processes to find the purest form of a thing as well as the transformation of matter. Although it was treated like a religion and full of secrecy due to the fear of being persecuted during the middle ages, there were still real scientific advancements that led to the chemistry we know today. In the 8th century an Arabic alchemist by the name of Jabir caught fire to vapor coming off of wines. Jabir was truly interested in science and was very good at documenting his work, which assisted later alchemists. Even with that discovery the distilled alcohol was still used primarily for medicinal and ritual purposes. As the centuries went by the process continued to be refined and the alcohol grew purer. By the 12th century we find writings in Italy as well as knowledge of the process spread throughout the known world. In 1500 we finally get the book, "The Virtuous Art of Distilling" written by Hieronymus Brunschwig with much more detail on how to make the medicinal "Aqua Vitae" (Latin for Water of Life) and by early 1600's there are writings discussing the frequent consumption of the Aqua Vitae. Though the path through history was hazy it got a little clearer as time went by, and there are many more names and places that could have been mentioned...we have finally arrived at drinking liquor. Now we all know that when we (the people) find something to enjoy and are happy to buy and sell to each other, it doesn't go unnoticed, cue the tax collector. Starting as early as the 1500's the British government wanted to tax alcohol, the higher the content, the higher the tax. One way they were testing the alcohol to decide if it was high content was to see if it would burn, using it to ignite gunpowder. Thus the term, it was "Proof" that it was high content liquor, and the content level needed to burn was dubbed 100 proof. Now this system was not entirely reliable and was later more specifically defined in legal documents in 1816 England where it was defined as having 12/13 the specific gravity of water at the same temperature until 1980. This made 100 Proof in England roughly 57.5% alcohol prior to 1980. The United States set it's "Proof" in stone in 1848 and defined as 50% ABV. Fun fact, this was done by starting with 50 parts pure alcohol and pouring water in until you had 100 parts solution. If you measured them separately to the same volume and mixed it would be less than the sum, in short, 1 cup of pure water and 1 cup of pure alcohol does not equal 2 cups. A little bit crazy science to think about but it happens because the smaller water molecules can fit between the larger ethanol molecules.
So now you know where proof comes from and generally in the United States "proof" on a bottle is double the ABV, a 40% whiskey is 80 Proof. So knowing that, what's the right proof? Well that depends on what you're drinking and how it's made. All types of alcohol can vary based on the distillery and brand but there are a few cut-offs, example, in Europe, a Vodka must be at least 37.5% to earn the name. There are many specifications around different Whiskeys and Bourbons but really anything aged in a barrel and not adjusted after will have inconsistent proof.
Measuring proof today is much easier as we have access to tools that keep it simple for you. There is a tool called a hydrometer that can help you test the proof of a liquor. A spirit hydrometer looks like a thermometer and floats in the liquid having lines along the side that can tell you what the proof of a liquor is. You can grab these all over amazon if you have an interest in testing. A liquor hydrometer is different that what is used on wines and beer which has to be much more sensitive to test the lower ABV. Kits usually come with a hydrometer as well as testing tubes and cleaners. Most if not all kits you buy actually have a hydrometer that shows both proof and percent, so it's a double win.
To test the proof or ABV you just poor some of your alcohol into the cylinder that comes with the kits. and put the hydrometer in. Once it finally gets still and stops bobbing, it will float steady in one spot. The indicator line on the hydrometer will show you what percent alcohol the liquid is. A couple of important things to know if you do this is that you must measure from the actual liquid level and not the meniscus which bevels up a bit from surface tension as shown in the diagram on the left. The second thing to keep in mind is temperature. The temperature will affect the reading and you can check against a proof-temperature chart. You can see in the picture below the chart and the effect that temperature has on the reading. I have uploaded the pdf file with the full chart document below if you need it and you can see the difference the temp makes. I hope this was informative!