How to Calculate Final Gravity of Beer
Gravity in beer is an interesting and seemingly complex subject, but it’s actually not that difficult to understand once you get down a few basics.
Armed with those basics, you can then determine the final gravity of your beer, which will help you determine the ultimate ABV.
First, it is important to know the role of yeast in beer, as it is the literal creator of the alcohol we want to calculate.
In beer, we have fermentable sugars and nonfermentable sugars, and those are critical to the work that yeast does.
As a single celled microscopic organism, yeast has a single role to play in nature – find sugars, consume them, and convert them to alcohol and carbon dioxide, along with hundreds of other micronutrients and enzymes that are largely good for the human body.
Yes. Beer, when made well, is healthy!
And it is all thanks to this process called fermentation.
With wine, the process is a very simple one – crush grapes, leave them out, yeast is attracted to the fermentable sugars in the grape juice (or must), and you have wine within a week.
With beer, the process is a bit more complex.
Beer had its beginnings in the rise of the cereal craze around 12,000 years ago.
You see, humans were mostly nomadic before that, but once they figured out they could settle down, grow crops, and irrigate the land, they also learned how to store their crops, grain among them.
Naturally, once you get to grow food, you also get to pick and choose how you eat it.
Roasting, kilning, and malting the grain became a new phenomenon, and various different meals were made from these toasted grains, namely cereal.
Well, all it takes from there is for a cereal meal to sit for too long out in the elements, yeast is attracted, and bam, you have beer within a week.
Beer, or mead or ale, began to be served around the world to family members of all ages as people realized that the fermentation process cleared the cereal water of all toxic substances that might have come from running rivers and streams.
The ABV in these ales were very low as a direct result of the low level of fermentable sugars in the wort as well as the wild yeast attenuation.
There are literally hundreds of strains of the Saccharomyces cerevisiae yeast, the yeast used for fermenting bread, beer, and wine, and each one has its own attenuation rate.
When you receive yeast from a manufacturer, the package will tell you the yeast’s attenuation rate.
The attenuation rate is how much of the fermentable sugars the yeast will consume during fermentation. That number is usually a percentage between 65% and 80%.
This is why, when you rack your beer for secondary fermentation, you can agitate your dormant yeast and get it to consume still more fermentable sugars in your beer, driving up the ABV in your final product.
You need your attenuation rate if you want to calculate your ABV by hand, using your original gravity and final gravity measurements.
Gravity in Beer
What do we mean when we talk about gravity in beer?
We are talking about how dense the beer is.
Why does that matter?
Well, think about it.
Water has a gravity of 1 at 4 degrees Celsius, so we always begin from a position of 1 because beer begins as water.
When we make beer, we add grain and hops to the water and boil and steep. When we remove the sediment, we have a ton of fermentable sugars in that water, now called wort. Sugar is much denser than water, so the gravity will go up.
The most common and easy way to take a gravity measurement is to use a hydrometer.
A hydrometer will give you a specific gravity reading both before and after fermentation, which is what you want.
For original gravity, you are measuring the density of fermentable sugars in your wort, so you will have a number like 1.050.
Write down your original gravity number, and then pitch your yeast.
Once fermentation is done, you will take a final gravity reading. Simply insert your hydrometer and write down your number.
The final gravity of your beer will be significantly lower than the original gravity of your beer because the yeast has consumed most of those fermentable sugars in your wort, and alcohol is much less dense than sugar.
How to Calculate
If for some reason you don’t have a hydrometer to measure your gravity, you can always calculate your final gravity by using the attenuation rate.
You will be making a rough calculation, determining how much sugar the yeast consumed based on what the manufacturer told you.
Final gravity = 1 + ((Total Gravity Points x (1 – Attenuation Percent) / 1000)
Do you have a headache yet?
Yea, you’re better off just investing in a hydrometer, because the real reason you want to measure final gravity is so you can figure out your ABV, which will be essential for your beer drinkers.
To calculate your ABV, all you have to do is subtract the original gravity from the final gravity and then multiply that number by 131.25, which factors in your alcohol by volume; the resulting number is your ABV%
In the end, it is much easier to figure out gravity and ABV if you understand what it is you are working with, and you have the right tools.
It pays to be prepared.
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