This calculation is based on Glenn Tinseth's research on hop utilization. The long and short of it is:
- More alpha acid = more IBUs
- More hop weight = more IBUs
- More time = more IBUs
- Higher gravity boil = less IBUs
This means extract brewers which use smaller boils will need more hops to get the same IBUs. There are some smaller adjustments we make in the utilization when the hops are addded to the mash or the first wort. Dry hopping does not add any bitterness to the beer but it does add aroma.