ABV

[Buzz]

Simple question - what equation do you use to measure the ABV in your beers.

Up until now, I've used the gravity drop divided by 7.46 (as per the very faded instructions that came with my boots hydrometer years ago). I see that in the Big Book Of Brewing, Dave Line also uses this equation.

However, I notice here that Jim uses the drop divided by 8.06. In the example of a beer OG1050 and FG1012, the difference is quite marked between 4.7% and 5.1%

I'm thinking my calculation may be too generous. My Winter Royal is a bloody nice brew but doesn't taste like a 6%er (OG1059 FG1014). Jims calc would put it at 5.6% which may be nearer the mark me thinks. Just interested to know what others use...

[MightyMouth]

I always use an online calculator as its just an estimation anyway as it depends on a lot of factors. Apparently (OG - FG) x 131 = ABV% is a good indication.

[oblivious]

Note: there are varying opinions on the exact value of the constants to use in the above formulae - mine err on the conservative side for alcohol content.

So Jim's beers are stronger than they appear, is the wife checking the alcohol content of the brews

[Buzz]

I always use an online calculator

Not sure I would trust that. It has my Winter Royal at 6.5% which it clearly is not. I'd say your OG-FG x .131 looks nearer the mark at 5.9%

[MightyMouth]

I always use an online calculator

Not sure I would trust that. It has my Winter Royal at 6.5% which it clearly is not. I'd say your OG-FG x .131 looks nearer the mark at 5.9%

I think the online one assumes you are priming with sugar maybe, I don't know.

[Calum]

I use ((OG-FG) * 0.129) in my brewing spreadsheet.

I either got this from one of Graham Wheelers book or the "Joy of Winemaking".

[MightyMouth]

That online calculator appears to be using (OG-FG) * 1.35 + 5) / 10) which is clearly wrong.

[Mashman]

I divide by 7.5 because I can just about manage that in my head.

Me too, I look at as a guide, I don't really mind if it's 4.5 or 5% as long as it tastes good.

[nobby]

First you have to presume the OG reading is correct. Its every easy to misread a Hydrometer also what temperature is it calibrated at. All make a difference so no point in being to accurate with your maths.

[Graham]

Trouble is that it isn't linear; alcohol has a low s.g., so it depends upon the ratio of alcohol and everything else as to the actual number we divide or multiply by. Quite a wide range of divisors will give results to good enough accuracy. The figure in my books OG-FG / 7.45 was calculated from first principles and held good for a 1040 beer, I think (might have been 1045), which I decided was an average home brew gravity. I will recalculate it some time in the future.

I believe that the Customs and Excise official figure is OG-FG *0.129 (which is the same as dividing by 7.75). However, there may be "frig-factors" introduced into the C&E official thing; cos they do things like that to keep errors where they want them.

I have tried to search various excise documents but can't find any reference to it. It must be there somewhere, it's just that I can't find it.

[steve_flack]

I have tried to search various excise documents but can't find any reference to it. It must be there somewhere, it's just that I can't find it.

from http://customs.hmrc.gov.uk/channelsPort ... P794_87219

30.3 Value of factor 'f' for various alcoholic strengths

(OG - PG) % ABV Factor

Up to 6.9 Up to 0.8 0.125
7.0 - 10.4 0.8 - 1.3 0.126
10.5 - 17.2 1.3 - 2.1 0.127
17.3 - 26.1 2.2 - 3.3 0.128
26.2 - 36.0 3.3 - 4.6 0.129
36.1 - 46.5 4.6 - 6.0 0.130
46.6 - 57.1 6.0 - 7.5 0.131
57.2 - 67.9 7.5 - 9.0 0.132
68.0 - 78.8 9.0 - 10.5 0.133
78.9 - 89.7 10.5 - 12.0 0.134
89.8 - 100.7 12.0 - 13.6 0.135

[Graham]

Value of factor 'f' for various alcoholic strengths

(OG - PG) % ABV Factor

Up to 6.9 Up to 0.8 0.125
7.0 - 10.4 0.8 - 1.3 0.126
10.5 - 17.2 1.3 - 2.1 0.127
17.3 - 26.1 2.2 - 3.3 0.128
26.2 - 36.0 3.3 - 4.6 0.129
36.1 - 46.5 4.6 - 6.0 0.130
46.6 - 57.1 6.0 - 7.5 0.131
57.2 - 67.9 7.5 - 9.0 0.132
68.0 - 78.8 9.0 - 10.5 0.133
78.9 - 89.7 10.5 - 12.0 0.134
89.8 - 100.7 12.0 - 13.6 0.135

Bugger! I must have spent an hour looking for that.
Well done!
So, 0.129 is good enough for a 1040-ish beer.

[Graham]

I'm assuming PG is the same as FG and it's not a typo as the p the f aren't anywhere near each other, so what does the P stand for ?

First column is the difference (OG - FG). Second column is typical ABV range (you can ignore it) Third column is the frig factor.

[Buzz]

Good work. I can sleep easy now I know my Winter Royal is 5.8%

[Graham]

Simple question

Surprising how many threads start off like that, and end up nothing like it.