I'm having a hell of a time trying to get a rough estimate of how many IBUs may be in an IPA I recently brewed, inspired by the NE IPA thread Recipe: 11.5lb Golden Promise 1.3lb Flaked Oats 0.1875lb Caramalt 15 Wyeast 1318 Batch Size: 5.5gal OG: 1.062 FG: 1.012 Hop Schedule: 1oz Galaxy (15.5% AA) - 5min 2oz Galaxy (15.5% AA) - 0min 5oz Galaxy (15.5% AA) - 30min whirlpool @ 155 I get anywhere from 65-120 based on the various free software on the net. Anyone with a guess that may get me closer to reality? I kegged it last night and it was quite bitter and green, but that could be because the hops had settled to the bottom of the keg before I could pull a sample. It's now dry hopping with 4oz galaxy.
I can't guess at the answer, but my suggestion would be to see what BrewCipher says. I based the models on data published in a paper by Mark G. Malowicki. I believe @telejunkie also has a model in his personal sheet. Maybe ask him to run it for you and then see how close the two answers are.
My own rule of thumb is to assume you get the IBUs as if you had boiled for half the time of the whirlpool. For example, 20 minutes whirlpool = 10 minutes in the boil; 30 minutes whirlpool = 15 minutes boiled; etc. Might not be 100% accurate, but it's in the right ballpark for certain. Might be more accurate to divide by 3 instead of 2 or something like that. More experiments are needed. But it's the right ballpark for sure.
Dave (@telejunkie) discussed this topic in his article published in BYO: “While hop forward beers can benefit from this technique, any beer where some hop nose is desired is also a good candidate. For low IBU beers, you can add no-boil hops or you can add a tiny bittering charge of hops to help break surface tension of the beer and then add all or the majority of the IBU contribution at knockout, with the 10–15% utilization in mind.” https://byo.com/stories/issue/item/2808-hop-stands Cheers!
A whirlpool at 155F in the above recipe will give a small IBU contribution. The OSU thesis by Malowicki says about 10%+ of a 212F boil, so more like 3% utilization as the OP proposed. My usual procedure if I want IBUs from the whirlpool is to put half in after flameout, then the rest at 170F. The hotter addition will give the 10-15% utilization, I set it to 12%.
