grav-i-ty – /ˈɡravədē/ – noun
Gravity is important. It keeps us from flying into space (which is a good thing) and it tells us how much alcohol is in our beer (which is an even better thing). If that second part doesn’t make sense right now, you know the routine, give me two more minutes of your time.
Gravity, in the beer world, is attached to three terms and concepts: Specific Gravity, Original Gravity and Final Gravity. Some breweries print these numbers on their beer labels and some bars may print them in their beer menus. All three of these terms refer to measurements taken throughout the brewing and fermenting process. Here’s a brief description of each:
Specific Gravity – The definition of specific gravity is “the ratio of the density of a substance to the density of a standard, usually water for a liquid or solid.” Water has a specific gravity of 1. If a liquid has a specific gravity of less than one, it’s less dense than water. If the liquid’s specific gravity is more than one, it’s more dense than water.
Original Gravity (OG) – The OG is the specific gravity measurement taken before the beer is fermented (when it’s called “wort”). This starting number tells us how many solids (sugars) are present in the unfermented beer, giving us a reference point for later. The higher the number, the more sugars are present which would allow for more potential alcohol.
Final Gravity (FG) – The FG is the specific gravity measurement taken after the beer is fermented. This number tells us how many solids (sugars) are no longer present because they have been consumed by the yeast. The lower the number, the more sugars have been consumed which would allow for more potential alcohol and a drier, lower bodied beer.
So, now that you know what these terms mean, let’s see how we get these measurements and apply them. There’s a special tool called a “hydrometer” that is only used to measure a liquid’s gravity. It has markings on its side and when floating in water, the top of the water will touch the line marked 1. When you put the hydrometer in the wort or beer to measure OG or FG, the marks that line the side of the hydrometer tell you how far above or below 1 you are. The difference between your OG and FG tell you how much alcohol is in your beer.
Let’s just throw a hypothetical situation out there to pull it all together. An American Lager, by style definition, can have a starting gravity between 1.040 (spoken as “ten forty”) and 1.050 (ten fifty). The final gravity of the style can be between 1.004 (ten oh four) and 1.010 (ten ten). Assuming this make-believe beer was right in the middle of both OG and FG at 1.045 and 1.007 respectively, we have all the info we need to calculate ABV.
Option 1 is to use this formula: ((76.08*(OG-FG)/(1.775-OG))*(FG/0.794)).
Option 2 is to click this link and plug in two numbers.
Good money says that if you started with the formula, you got two or three different answers, cursed under your breath about high school and Ti-83’s and then angrily clicked the link. Better money says you just kept reading and didn’t do any math or link clicking. Regardless of which way you did it, your answer should have been right around 5.0% ABV.
If you’re thinking there’s more than one way to calculate ABV, you’re absolutely right. There are plenty of ways to get to the same conclusion with all kinds of instruments and more complex calculations but we’ll leave that to the professionals. That all sounds pretty heavy to me.
C’mon. An entire gravity article without at least one gravity pun would be a travesty. At least I weighted until the end.
Do you have a beer term that you’d like to know more about? Email me at email@example.com or comment below with something you’d like to see explained in a future Vocabrewlary Lesson.