Hops and amount of wort boiled

My recipe says that the hop utilization is for a 2.5 gallon wort boil with an additional 2.5 gallons of water added before pitching yeast. I was planning on a 5 gallon wort boil. Will that really affect the final IBU.

Yes. Your hop utilization increases the lower the gravity of your wort. If you are doing a 5 gallon boil your gravity it going to be much less than the gravity of a 2.5 gallon boil. If you are doing an extract brew then you would get better hop utilization by using about 25% of your extract for the full boil and adding 75% of the extract with about 15 minutes left in the boil. If you have access to a program like Beersmith you can do the exact calculations. If you want to give me the recipe I can plug it into Beersmith for you.

Imperial blonde ale from a kit. Fermentables are 6.6 lbs light lme,1lb. wheat dme and 1lb. corn sugar. Grain is 1 lb honey malt. Hops are 1.5 oz. brewers gold bittering for 40 minutes of boil, .5 oz cascade flavoring with 20 minutes left in boil and .5 cascade for aroma last 10 minutes of boil. Yeast is wyeast am
erican ale 1056.

Ok, I plugged this into Beersmith with a 5 gallon boil for 40 minutes. This calculation factors that you steep the grains for 30 minutes at 155 degrees and that you boil the corn sugar and wheat dme for 40 minutes and then add the LME and boil for 15 minutes.
OG - 1.066
IBU's - 53.1
SRM - 9.5
FG - 1.012
ABV - 7.1%

If you boil all of your extract for 40 minutes the IBU's drop to 39.6. So you can see how the gravity of the wort affects your hop utilization.

Boil time will also affect your bittering hop utilization. If you increase your boil to 60 minutes your IBU's go to 59.1. Again this is if you add the bulk of your extract in the last 15 minutes of the boil.

Having a tool like Beersmith allows you to plug in different variables so you can design a beer with the hop and malt character you are looking for. I am not too familiar with the Blonde style so I don't know if you are trying to accomplish.

Good luck!

Thanks for the numbers and quick response. I appreciate it.

No worries. I hope your brew turns out awesome.

Those calculators aren't very accurate when figuring IBUs in partial boils. I'd just make the beer with the full wort volume and not worry about the hops utilization.

John Palmer himself has come out and said that he got it wrong in How to Brew- that IBUs are not dependent on wort gravity. There are some new experiments done by Basic Brewing Radio showing the same thing.

So don't worry about reducing hops or adding hops based on the size of the boil. It'll be fine.

@SSD - How'd you like the snow this morning?

Snow was great til I had to shovel it. Got about 8 inches at my house.

Thanks for the information everybody. I've had two different responses about IBU's, guess I'll flip a coin and keep my fingers crossed.