Beersmith2 IBU Calculator

I brewed EdWort's Haus Pale ale this weekend exactly as he has it listed in his thread. In his thread, it says it should be 39 IBUs with a 6.5 gallon pre-boil volume and a 5.5 gallon final volume. When I enter this recipe into BeerSmith2 it gives me 31.2 IBUs. I noticed when you change the 'equipment' option it changes the IBUs. Why would the pot have any effect on the IBUs if you still have the same pre and post-boil volume? Can someone explain this?

Excess headspace can allow space for essentials to evaporate from the wort....there are also factors of boil efficiency based on the geometry of your kettle and heat source(s). Materials matter too - copper is very different from stainless (let's just not mention enamel).

Check the IBU calculation, Rager or Tinseth will make a difference.