It is pretty easy to calculate the impact that diluting the wort will have on gravity with most of the brewing software and calculators available, but I was wondering if anyone knows of any calculators that can calculate the impact that diluting has on the IBUs. For example, I brewed a batch of a zombie dust clone yesterday in which I overshot my mash efficiency by over 10% but undershot my brew house efficiency (from a volume standpoint). I didn't have enough time to allow my kettle to rest for a while after whirlpooling, so I ended up leaving over a gallon of wort in the kettle with my hop material and cold break. As a result, I ended up with 4.5 gallons of 1.072 wort when I was targeting 5.5 gallons of 1.063 wort. Brewcipher estimated the IBUs to be 69; since I_overshot my OG by so much I added a little less than a gallon of water to bring the volume up to almost 5.5 gallons and the OG down to 1.060. My question is how can I determine the impact to the IBUs? Can I assume that this will drop the IBU's by roughly 15%, which would equate to a decrease of about 10 IBUs?
I'm not sure if you recalculated your IBUs for a 1.072 post-boil wort, but if so, this will get you in the ballpark... 69 IBUs x 4.5 gallons = 310.5 IBUGallons* 310.5 IBUGallons / 5.5 gallons = 57 IBUs *IBUGallons is a unit invented for convenience.
