I have read that whole hops impart less bitterness than pellet hops by weight. I have used pellet hops for all of my brews thus far. I have a recipe that I am happy with, and I want to switch to using whole hops. Is there some reliable conversion between appropriate weights for whole hops vs. pellet hops? Is it perhaps 20% more — or some other number?
You will get more utilization from pellet hops since they are pulverized, giving them more contact with the wort during the boil. However there isn't a hard and fast conversion rate, since there are a lot of variables with hops. Brew Dudes suggests you get 10-25% better utilization with pellet hops, and 10% is the typical number I've heard, so working off of that, you can use about 10% more whole hops to make up the difference.
I'm not sure what scale you are working on, but instead of buying another ounce of hops to get an extra .2-.4 ounces, I might just increase the boil time for the existing hops to bump the IBUs.
I'd say just experiment with slightly more hops or a slightly longer boil until you get something that tastes about right, but unless you have a sensitive palate you may not even notice the difference.