Calculating ABV

KwaiLo · Post by **KwaiLo** » Tue Dec 13, 2011 1:08 am

I brewed a nice big barleywine the end of November, OG was 1.124. We pitched S-04, and had a nice big fermentation. 14 days in primary, we were down to 1.022, giving us a 13.82% ABV. Maybe. Or, it might be 15.38% ABV.

Most calculators use;
ABV = (og – fg) * 131.25

Seems that there is also an alternate formula;
ABV =(76.08 * (og-fg) / (1.775-og)) * (fg / 0.794)

With the non-linear relationship between the changes in gravity, the rounding of digits before the end of the equation throws the number very far off in higher gravity beer. A 4% beer might be out by .02% ABV, but a 14% beer might be out by 2%. Big, big difference.

My issue is that the standard ABV is so far divergent to the majority of the beer I brew as to be almost useless. Unless of course, the rest of the brewers I speak with, share beer with etc etc use the standard method.

What method do you use?

grub · Post by **grub** » Tue Dec 13, 2011 11:37 am

promash tells me 13.72%
brewzor tells me 13.59%

so it looks like they're both using the simple one (and some variance due to rounding, i assume), so i guess that's what I use. wouldn't sweat it much either way.

icemachine · Post by **icemachine** » Tue Dec 13, 2011 11:59 am

The more complex one is supposed to give better accuracy for higher gravity beers. Here is an explanation of why they differ

http://www.brewersfriend.com/2011/06/16 ... r-updated/

markaberrant · Post by **markaberrant** » Tue Dec 13, 2011 12:15 pm

It's just a calculation based on an educated guess. I wouldn't worry about it too much, same with IBU and SRM calculations. All that matters is how it tastes.

KwaiLo · Post by **KwaiLo** » Thu Dec 15, 2011 1:16 am

Thanks guys. I prefer to be accurate, or at least more so, especially given there is no more effort to do so.

markaberrant · Post by **markaberrant** » Thu Dec 15, 2011 9:19 am

KwaiLo wrote:Thanks guys. I prefer to be accurate, or at least more so, especially given there is no more effort to do so.

All I am saying is that hydrometers are notoriously inaccurate and the calculations are just an approximation.

If it is truly important to you, then send it away to have it analyzed in a lab. I have done this a few times, and it was very interesting how off my "accurate" estimates were. Just look at all the product coming into Ontario that gets properly tested and is wildly different in terms of the brewery's ABV claims and the lab results.

High gravity beers are going to be more inaccurate in terms of ABV calculations, and high IBU beers are also going to be inaccurate. The math built into these formulas gets wonky and doesn't scale.

So don't sweat it. Sure I use my hydrometer, but if it finishes at 1.012, and I was hoping for 1.009, I don't really care. Let your tastebuds be the judge!

Peter Collins · Post by **Peter Collins** » Fri Dec 16, 2011 7:59 pm

Use both and average them out?

Isn't that what we did today with our pre-boil readings?

The other solution I use is: Did you end up with beer? Mmmmmm... beer.

13.8% vs 15.4%? Try two pints and then see if you care what the actual reading is. LOL

Sorry, just being my normally useless and quasi-witty self.

Derek · Post by **Derek** » Sat Dec 17, 2011 4:43 pm

I'm a scientist and I haven't found any software that I'm really happy with... so I use a mix of different things.

Brewheads has a nice calculator, it seems to use the full formula, and you can enter your gravities in Brix as well:
http://www.brewheads.com/stats.php

Using that, I find that the difference in my specific gravities, as well as the difference in Brix, result in similar abv's.

That said, they're still just calculations and big brews do seem to come out weaker than the calcs (look at some of the LCBO testing). So maybe the 'weaker' formula is actually more accurate?

One of these days I'll right my own software... or at least put a bunch of Java scripts together.