O is for Original Gravity

O is for Original Gravity


No, this is not a measure of Earth’s gravitational force when it first appeared.

Gravity in this case is in reference to density. We compare the density of the brew we’re making with that of water. We do this to determine approximately how much sugar is in the brew before fermentation and then to determine the alcohol content after fermentation is done.

The unfermented beer has a density greater than that of water. Water has a defined density of 1. Thus, an unfermented typical beer might measure around 1.040. The higher the number, the more sugar. Some heavy beers, like stouts or the Belgian I brewed earlier this year, start with a density of 1.080.

This starting density is called “Original Gravity.”

We measure the density again when fermentation is complete. Alcohol has a density less than that of water, so the gravity of the beer drops, often to something like 1.010. This is the “Final Gravity.”

We can compare the original gravity (OG) to the final gravity (FG) to calculate the alcohol content using this equation:

Alcohol by Volume (ABV) = (OG – FG)*131.25 or (OG – FG)/0.75

Most beers I brew come out around 4% ABV. A few are in the 2% range. The Belgian is about 8%. One of those is enough to get me through the night!

I’m participating in the April A to Z Blogging Challenge. The goal is to write a post prompted by a letter of the alphabet on each day of April (except Sunday). My theme this year is brewing. Visit my other A to Z posts by clicking here.

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