Extract Recipe Calculation Question

[Mixmasterb]

Hi all! Long time reader first time question asker

I am creating an extract recipe with specialty grains. Basically I am using the same recipe as I got with a kit so I know the recipe itself is OK, my questions are more to do with the recipe calculator and my hop additions.

I am planning to boil 3.5 gallons and then top off to 5 gallons. I am assuming I will lose 1 gallon to evaporation. I will add about half my extract at the very end of the boil.

When using a recipe calculator I get vastly different IBU's when calculating whether I boil 2.5 gallons vs 3.5 gallons. How does evaporation effect this? Do I calculate based on the 3.5 gallons since that is what I will actually be boiling and so it doesnt matter that it will end at 2.5gal and be topped up to 5?

The second part is do I calculate my hops based on the lower gravity that doing half my extract for the full boil and so the late addition won't really affect it, or do I calculate based on the whole extract amount since that is what I ultimately end up with?

Sorry for the long post, just trying to be clear. Thanks for the help.

 

[maffewl]

Calculate your hop additions based on your beginning boil size. Once you get your process dialed in, you will be able to calculate off batch size as the boil size will be an "auto-calculation" and you can really calculate off of either. As for the malt calculation, base it off of the OG for the majority of the boil. Or to be more accurate, calculate based on the exact times for the malt additions, such as extract A (50% of total) added and boiled 60 min, and extract B (50% of total) boiled for 15 min.