Last week I talked about using a hydrometer or refractometer for obtaining your OG (original gravity) and your FG (final gravity), now I will let you tell you how brewers figure out the ABV (alcohol by volume) of their homebrew with these to readings. The number you get when you subtract your FG from your OG is the number you will use when calculating your ABV. One must remember that when you are taking your FG reading that you should take at least 2 readings a few days apart to make sure that the fermentation is complete. Once you have obtained two identical readings you will have your FG reading.
Many brewers keep a log book or an excel spreadsheet for recording all of their brewing activities and readings. There are many different brewing logs sheet available online, or if you purchased a beer recipe kit it may come with something to record your brewing process. It is a good idea to keep a journal of all your brews, not just to record your OG and FG readings, but everything you used and the amount of time each step took.
Once again I have taken a look at many forums and found that there are quite a few different numbers used to find the ABV, although they all are close. I will give you 3 examples that I found using the following readings - OG = 1.075 FG = 1.010
1) 1.075 - 1.010 = .065
.065 x 131 = 8.515 ABV
2) 1.075 - 1.010 = .065
.065 x 132.715 = 8.62 ABV
3) 1.075 - 1.010 = .065
.065 x 131.25 = 8.53 ABV
I also used some online ABV calculators using the same numbers and got the following results:
1) 8.53 ABV
2) 8.70 ABV
3) 8.53 ABV
So ... 50% of the calculations I tried gave me the answer 8.53 ABV. If you are looking for a quick way of finding our your ABV I would use the calculation that multiplies 131.25 times the difference between the two readings.
You will hear and see SG (specific gravity) mentioned quite often when brewers are discussing ABV, so what is it exactly? The term SG is interchangeable with many different types of readings, even OG and FG - the reason being is that SG is the comparitive measure of relative density between two similar materials. That is why using a special brewing measuring tool to determine your SG is very handy so that you do not have to figure out what the SG of your wort is and then divide it by the density of water (which is the comparision liquid in brewing). Using a hydrometer or refractometer for your OG and a hydrometer for you FG you will be able to quickly look at the scale on your meter to get your SG readings for both.
Next week: I am ready to bottle my homebrew - now what?
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