View Full Version : Mead Calculation question: Delta SG--->ABV

09-26-2006, 10:23 AM
Maybe this is the wrong spot to post this..General Questions might have been a better choice, but I see it as related to the Mead Calculator, so I put it here. Feel free to move it, and my apologies for creating the extra work.

For all practical purposes, the current approximations are fine, but I am a little perplexed as to why searching the web on various Winemaking sites give different values for the constant to convert OG-FG to % ABV.

It seems the Mead Calculator here uses a multiplier of 135-136, which is in accord with one value of division by 7.36 (a multiplier of 135.87)

MOST other sites give multipliers of 131, while others are around 126,

and this calculator from http://powersbrewery.home.comcast.net/tables.html

seems to use a value of 106!

Inquiring minds want to know why there is such diversity in the reporting of what should be a fairly well understood physical constant.



09-26-2006, 11:51 AM
Best description I found once from an English forum:

(OG - PG) x f = a% ABV

where OG is the original gravity of the beer;
PG is the present gravity of the beer;
a is the beer's alcoholic strength; and
f is the factor connecting the change in gravity to alcoholic strength.

The value of "f" is not constant because the yield of alcohol is not constant for all fermentations. In lower strength beers, more of the 'sugars' available for fermentation are consumed in yeast reproduction than in producing alcohol.

5 The table below produced by the Laboratory of the Government Chemist shows the changing value "f" depending on the alcoholic strength of the beer.

(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

The table indicates that for the majority of popular UK beers, the factor for calculating the estimated strength would lie in the range 0.128 and 0.129. For strong beers (exceeding 6% ABV), the factor would be in the range of 0.131 - 0.133. Whilst it is important that the correct value for "f" is used, you should be aware that this table is primarily for your guidance. Some brewing methods and materials can affect the factor.

So, the value used for the calculation is dependant on alcohol content. Each calculator available may therefore use a slightly different value depending on who chose the constant and whether they selected it based on beer making or wine making experience.


09-26-2006, 12:02 PM
Thanks!! This is just the kind of analysis I was looking for.

Most places treat 'f' as a true constant, and I wondered if it was or not!

I still wonder why the Beer Calculator has such a low value, which is STILL Low with the expected %ABV of a beer from your table...but I don't really care :-)

Thanks again for the great reference!


09-26-2006, 01:08 PM
Good info.

If beer, mead and wines was only alcy and water, then maybe the gap bewteen the factors might be affected by the types of alcys. I bet one could look up the common alcy's that are produced during fermentation and I wouldn't doubt that the SG of each (in a purer form) would be different. If that is so, the blend of those alcys would affect the overall SG of the mix. And I wouldn't be surprised if that blend can also change at different stages during the feremnetation

And it might have something to do with the extra stuff (minerals and the like) in there as well. With that in mind, perhaps a mead would have a different factor than an equal origanl/finish gravity wine. Similarly for beer.

I'm just speculating at this point but wouldn't be surprised if something like the above was the reason.