ABV Calculations

[splitter]

Just wondering what the general consensus was regarding formulas?
Asking because of large differences between diferent formulas i have seen.
For e.g.
a drop from 1040 to 1010 by Jim's calculations =
(1040-1010)/8.06 = 3.72%
and by the formula with coopers kits =
(1040-1010)/7.46 + 0.5 = 4.52%
Seems like a big difference to me.
The extra 0.5% in the coopers was supposed to be for priming sugars.
All opinions recieved with interest.....

[Wez]

I've been told to subtract FG from OG and divide the difference by 7.45

[Jim]

Yep, the general concensus seems to be that my formula is a bit pessimistic. I based it on some information I found (on the web) that worked it out from the actual chemical reactions, molecular weights, etc. to give the correct theoretical answer.

Since then, I've read some other stuff that suggests that the theoretical answer doesn't work so well in practice, and that you actually get a slightly higher figure - hence the footnote I added to that page.

At the end of the day, though, when you think of the (lack of) accuracy with which we measure sg (unless you have a properly calibrated hydrometer or refractometer) a ballpark figure is good enough.

[Wez]

A good test is to attempt to drink ten pints of the brew and see at what point you wet yourself/get into a fight(maybe even with your own shadow)/put your pants on your head and tell SWMBO it's her lucky night that should give you a reasonable indication of how strong it is. IMO

[bitter_dave]

[steve_flack]

FWIW, the one used in BeerAlchemy is based on formula 4d in this link and it works out the ABV via the ABW. As you can see from that page there's many different ways of doing it.

[SteveD]

I use SG drop divided by 7.4 That gives ABV's that fairly closely match the OG figure on beers that have gone to quarter gravity. The poetry of that is good enough for me. Eg 50-12.5 = 37.5/7.45 is 5.06% ABV

Wheeler uses 7.4, and he is God. So there.

[splitter]

Thanks for all the replies guys. Interesting stuff.
Just trying to gauge the general opinion.

At the end of the day, though, when you think of the (lack of) accuracy with which we measure sg (unless you have a properly calibrated hydrometer or refractometer) a ballpark figure is good enough.

Definately have to agree with you there Jim.
I think i'll have to go with wez's 10 pint method,
using my well calibrated liver!
Cheers.

[oblivious]

I use SG drop divided by 7.4 That gives ABV's that fairly closely match the OG figure on beers that have gone to quarter gravity. The poetry of that is good enough for me. Eg 50-12.5 = 37.5/7.45 is 5.06% ABV

Wheeler uses 7.4, and he is God. So there.

I also use that system