Home brewers talk a lot about gravity. It's not the type of gravity that keeps us on the Earth, but the gravity of their beer recipe. This includes the beer's specific gravity, original gravity, and final gravity.
Brewers measure gravity throughout the brewing process to estimate how many sugars can be converted to alcohol. In the end, these calculations are used to determine the beer's final strength (its alcohol by volume or ABV).
Crucial in this process is understanding the recipe's original gravity. Without it, the ABV cannot be determined.
What is Original Gravity?
Original gravity (abbreviated OG) is a measure of the fermentable and unfermentable substances in a beer wort before fermentation. Those substances are often the sugars that will be converted to alcohol during the fermentation process.
OG gives the brewer an idea of the potential alcohol content in the final product. After fermentation when OG is mathematically compared to FG (Final Gravity), the exact alcoholic strength can be determined.
For example, Kyle's Nut Brown ale had an OG of 1.054 and an FG of 1.015 resulting in 5.3% ABV.
How to Estimate Your Home Brew's OG
Before you can get an OG, you must get an SG (specific gravity) reading by using a hydrometer or similar instrument. The SG compares the density of the beer (or wort) to the density of water.
Water has a specific gravity of 1.000. When grains for the wort are added, the density increases. For example, a stout may have an SG reading of 1.090, which means that it is 9% more dense than water and you can assume that it contains 9% sugar.
Tip: SG will be used in the beginning and the end of the brewing process. You will have an 'original specific gravity' (OG) and a 'final specific gravity' (FG). In the end, the difference between the two gives you the ABV.
Find Your PPG
When buying grains or malt extracts for a beer recipe, you will need to know each grain's PPG rating (points per pound per gallon). The supplier may have this information and there are online charts available for common grains.
- PPG can be found by adding 1 pound of grain or malt extract to 1 gallon of water and taking a hydrometer reading.
- The reading will look something like this: 1.034
- The PPG is 34 (the long math is Hydrometer reading x 1000 - 1000 -- though you can just use the last two digits).
Once you have your PPG, you can calculate the OG:
Original Gravity = Amount of Extract x PPG / Batch Size
Mash efficiency must also be factored in. Efficiency is determined by how well your equipment and brewing process extracts sugars from the malts during the mash/lauter process.
In reality, the OG calculation looks like this because no homebrew operation is 100% efficient (most are in the 80% range).
Original Gravity = Amount of Extract x PPG x Efficiency / Batch Size
Using OG and FG to Calculate ABV
The formula for estimating the alcohol by volume of a beer is easy when you have the two gravity numbers:
ABV = ((( OG - FG) x 1.05) / FG) / 0.79
These two numbers will remain consistent across all beer recipes:
- 1.05 is the weight of ethyl alcohol
- .079 is the density of ethyl alcohol
Online Beer Calculators
Did you think that brewing beer would require so much math? It is a scientific process and does require a lot of careful calculations and planning, but you don't have to do the math yourself!
The internet has opened up the conversation for home brewers and it is easier than ever to get help with your beer recipe. Most home brewers will find online calculators to be vital to their success.
- Brewgr - This website is a great resource for understanding all of the calculations you need for brewing your own beer and the Homebrew Recipe Calculator is easy to use. There is also a great collection of proven beer recipes to explore.