I’m a beginner brewer and have been reading “How To Brew” over the past few days. I think I have grasp on how to calculate IBUs, but I do have a few questions.
First: I’m starting off with extract kits; the next kit I’m going to try is the Black IPA. According to Northwest’s instructions for this kit, the OG is 1.075. This kit uses a total of 9.15 lbs of dark malt syrup. Should I assume that gravity of the syrup is around 1.036, as stated in the book? Also, the recipe calls for a 1 lbs of corn sugar at knockout. Is there a way to calculate the approximate gravity before boiling?
Second: When I was calculating the IBU’s for this kit I came up with what seems to be a high number, 81. Does this sound right?
Hops:
Summit (60 min) - .184 U - 50 IBU’s
Chinook (15 min)- .092 U - 18 IBU
Centenial (10 min) - .067 U -10 IBU
Cascade (5 min) - .037 U - 3 IBU
Centenial (0) - 0 U - 0 IBU
CasCade (Dry Hop) - 0 U - 0 IBU

I will be using the hop recipe that is called for by the kit. The boiling times posted are also per the recipe. I calculated the AAU’s based on a average % for the hops and determined the Utilzation from those averages.

To be honest I didn’t use either equation. I’m not very good at algebra/math; John Palmer, author of “how to brew”, provided a 'dummied down" equation for those of us who aren’t strong in mathematics. Palmer does state that his equation isn’t as accurate as the Tinseth formula or the Rager formula; but he feels that there is only a 1-3 IBU difference. the equation that Palmer provides is as follows: IBU=AAU x Utilization x 75/Volume of Recipe.

I don’t plan on deviating from from the NB recipe, as I mentioned I’m a rookie brewer and am still on my first batch. But, I’m trying to grasp a better idea of the process/science of brewing and was hoping that I could apply what I am learning in the book to better understand what is taking place while I am making an extract kit.