Originally Posted by xmacro
Sorry if this sounds dumb, but if the recipe calls for a 2.5 gallon boil and I go with 5 gallons instead, should I divide the hops by half or double the amount I use? Using double the water means there'll be more in solution, so I'd think I'd have to cut the amount in half - is this right?
Make the recipe as written. If you're making 5 gallons of beer, it's still 5 gallons in the end. You don't lower the amount of chocolate chips in a cookie recipe if you add them later. For a 5 gallon batch, follow a 5 gallon batch recipe and don't change anything.
Hops utilization is NOT impacted by gravity, and even if you do get slightly better hops utilization that's fine because it's an IPA. Boil size impacts hops utilization due to topping up, but it's not all that much. It used to be thought that wort gravity impacted hops utilization, but since 2008 or so that has been proven to be untrue.
In some very low IBU beers (say, a 15 IBU cream ale), you may want to consider really thinking about bittering hops since a few IBU difference may be noticeable. But not in an IPA/APA/American amber/etc.