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The formula I've consistently found for calculating the IBU for a particular hop is:

IBU = Weight (ounces) x Alpha Acid % x Utilization % x 7489
      -----------------------------------------------------
                            Volume

However I've read in several places that, for OGs north of 1.050, there is a so-called gravity correction factor that gets applied to that equation, and I am looking for clarity on it.

What is this gravity correction factor, how does it impact the above equation, and why is it needed? Why is 1.050 the "threshold" for using it?

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I consulted "Designing Great Beers", and indeed, it shows the same formula, without really explaining why the gravity correction factor is applied, only that it needs to be applied, with emphasis from the author, at wort gravity greater than 1.050 "during boiling". So not because your end product has a gravity greater than 1.050.

The reason is that alpha-acids isomerize with more difficulty in worts that have a gravity greater than 1.050. So your utilization drops, and you need more hops to obtain the same bitterness.

You should actually apply this to the whole boil. You need to check at every point where you want to add hops (e.g. at 30 and 15 minutes before the end of the boil) what the gravity is at that point and adjust your utilization accordingly.

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    Thanks @chthon (+1) I found a web-based explanation of this method here, which includes the actual formula for it. Everything makes perfect sense, thanks again! Oct 3 at 10:53

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