ABV Calculation

I have been brewing using ingredient kits. The directions that come with the kit say the formula for determining ABV is : (OG - FG)*131.25 = ABV.
I just found another one that says the formula is: (OG – FG)*1000/7.36.
Any thoughts on which one is the correct one?

Do the math, both work fine. I use the one which multiplies by 131.25.

(1.036 - 1.011) x 131.25 = 3.3%
[(1.036 - 1.011) x 1000] / 7.36 = 3.4%

ETA: It works kind of like 20% of something can be found by multiplying by 0.2 or dividing by 5.

I,of course, did the math. The higher the ABV the more the difference. I had a brew come out at 6.8% the first way and the second one came out at 7.0%. I just wondered if there was an "official" formula.

I believe the
(OG - FG)*131.25=ABV
Is the more commonly accepted equation for ABV

Or just use a calculator.. http://www.brewersfriend.com/abv-calculator/

Neither of those formulas is correct. It's not a linear relationship.
The 131 number is an approximation for beers that end around 1.015. The other is an approximation for ending around 0.99 - eg wine.

billl said:

Neither of those formulas is correct. It's not a linear relationship.
The 131 number is an approximation for beers that end around 1.015. The other is an approximation for ending around 0.99 - eg wine.

Click to expand...

Hmmm, so how do we know how strong or weak our beer is? Too bad there isn't a meter you can just dip in the beer for a ABV reading.

If you *really* want to know, you could send it to a lab for testing, but most of us accept that what we figure us close enough!

I mean, the difference between 6.8 and 7 is pretty small, even if it's not exact.

Brownalemikie said:

Hmmm, so how do we know how strong or weak our beer is? Too bad there isn't a meter you can just dip in the beer for a ABV reading.

Click to expand...

Well, there is. You could go drop a couple grand on a spectrometer if you REALLY want to know for sure. You can measure alcohol content quite accurately by the wavelengths of light it absorbs.

Anyway, The relationship isn't unknown, it's just not linear. ie some constant times your gravity reading. There are plenty of online calculators out there.

There's no easy calculation. As far as I'm aware, all the other intermediaries that can be produced during fermentation mean you'll likely never have a calculation to tell you exactly. Has to be measured to know for sure.

(OG-FG)*131.25= is the quick napkin math version based on SG I've used in the past.

(76.08*(OG-FG)/(1.775-OG))*(FG/0.794)= is the longer equation I used to use on my spreadsheet, allegedly more accurate, but both equations are very close at lower ABV levels, but less so as you get higher (the longer one allegedly more accurate).

Lately I use ((OG-(0.1808*OG + 0.8192*FG))/(2.0665-0.010665*OG))/0.789= which originates from Balling, but is based on %extract (ie °Plato), not SG.