Thu Sep 15, 2011 5:14 pm
Since you don't need to know alcohol calories (although a small amount may remain), you only are going to want to calculate your carbohydrate calories.
Carb calories per 12 oz bottle = 4.0*(RE -0.1)*FG*3.55
Where RE (real extract) = 0.1808*Pi + 0.8192*Pf
Where Pi (degrees Plato initial) = -463.37 + 668.72*OG -205.35*OG^2
Where Pf (degrees Plato final) = -463.37 + 668.72*FG -205.35*FG^2
You can get an approximation of degrees Plato by dividing the last two digits of your specific gravity by 4 instead.
If you had a beer that started with an OG of 1.052, and finished at 1.012, using the approximations for Pi and Pf, you would have 13 P and 3 P respectively.
RE = 0.1808*13 + 0.8192*3 = 4.808
CarbC = 4.0*(4.808 -0.1)*1.012*3.55 = 67.66 calories
Using the long calculations for Plato, you would get
RE = 0.1808*12.86 + 0.8192*3.07 = 4.84
CarbC = 4.0*(4.84 -0.1)*1.012*3.55 = 68.1 calories
So the Plato approximations are probably close enough.
If you want to know calories due to alcohol, you can get them from the formula
6.9*ABW*FG*3.55 = EthanolC
Where ABW = [((OG-FG)/0.75)*.79]/FG
Adding EthanolC and CarbC will give you total calories for the beer...
I think that the new version of How to Brew may have a simpler form of these calculations, I couldn't find it on the online version. I modified these formula based on the work of Peter Ensminger.
Spiderwrangler
PFC, Arachnid Deployment Division
In the cellar:
In the fermentor: Belgian Cider
In the works: Wooden Cider