Original Gravity: Homebrew Science

If you’re into the homebrew scene, you might hear a lot of talk about gravity. Of course, this doesn’t refer to the Earth’s gravity, but rather the gravity of your beer recipe. The most important of these is the beer’s original gravity, which plays a crucial role in how you beer will turn out…

Original Gravity: Why It Matters

What it is

The original gravity (OG) in brewing is a measure of the fermentable and un-fermentable substances in the beer wort before it ferments. This seems like an odd thing to measure as a brewer. However, it actually is important for figuring out the ABV of your brew.

Still, you can’t do this with the OG alone. You’ll also need your beer’s final gravity as well. Final gravity is calculated after your wort has finished fermenting. Then, you’ll take these two numbers and compare them to find your beer’s ABV.

The PPG

Finding out your original gravity will involve having to do some math. Thankfully, it isn’t all that difficult. Firstly, you’ll need to determine the points per pound per gallon (PPG) of your grain. Often times, suppliers tend to have this information available online or on the packaging.

Once you have this number, then you can find the OG by multiplying the amount of extract by the PPG, and dividing that number by the batch size. Still, don’t forget to consider how efficient your batch is, as most are around 80%. To get a closer OG, multiply your estimated efficiency along with the PPG and extract, and then divide by batch size.

Calculating ABV

After you have found out your original gravity, then it’s time to brew the beer and calculate it’s final gravity. Once you have these two numbers, you can find out your brew’s ABV. Due to the math you did earlier, this calculation is now very simple.

First, subtract your FG from your OG. Then, multiply that number by 1.05 (which is the weight of ethyl alcohol). Next, take that new number and divide it by your FG. Lastly, divide this number by .79 (the density of ethyl alcohol), and you’ll end up with your beer’s ABV.