Expert: Ed Westemeier - 12/2/2007

Question
How do you calculate the alcohol content from specific gravity?

Answer
There are a number of formulas, but for all practical purposes you can get a simple answer that is close enough by using this one (I'll simplify it as much as I can):

Take the SG reading at the time you pitch the yeast in the wort. Call that A.
Take the SG reading when fermentation is completely finished. Call that B.
Subtract B from A. Call the answer C.
Multiply C by 1.05 and call the answer D.
Divide D by B and call the answer E.
Divide E by 0.79 and the answer is the percentage of alcohol (by volume).

Here's how it works:
Assume the original gravity was 1.048 (standard for most commercial wort), and the final gravity was 1.012 (a typical value).
1.048 minus 1.012 equals 0.036 (there's our C).
0.036 times 1.05 equals 0.0378 (that's D).
0.0378 divided by 1.012 equals 0.03735 (roughly), which is E.
0.03735 divided by 0.79 equals 0.04728 or a little over 4.75% alcohol.

That's about as simple a formula as you can find.