[metalhophead]

I have seen lots of topics about this, but I still am unable to get a definitive answer. The recipe I am doing calls for a 6 gallon boil, but I only have a 5 gallon pot right now. I want to know what is the best way to get the correct IBU's with a partial wort boil. I am think doing a 4 gallon boil while adding 1/3 of the malt extract in the beginning and the rest with 15 minutes left makes the most sense, but is there a way to calculate the IBU's somehow while doing this method? I am trying to clone Dogfishead's 60 minute so I need the IBU's to be right. I've considered the Texas two step, but I feel my idea is easier. Any help is greatly appreciated.


Here is the recipe:

Here it is:

Pre-boil tea at 150°F (66°C)
6 gallons (23 L) water
Grain bag
6 ounces (75 g) crushed British
amber malt


Boil
7 pounds (3.2kg) light dry malt extract
(75 minutes)

1/2 ounce (15g) Warrior hops
(Add gradually over 60 minutes)

1/2 ounce (15g) Simcoe hops
(Add gradually over 60 minutes)

1/2 ounces (55g) Amarillo hops
(Add gradually over 60 minutes)

1/2 ounces (55g) Amarillo hops
(End of boil)

Fermentation
Yeast: Wyeast 1187 Ringwood Ale

1 ounce (55g) Amarillo hops
(6 to 7 days)

1/2 ounce (15g) Simcoe hops
(6 to 7 days)

Bottling
5 ounces (125g) priming sugar

STARTING GRAVITY: 1.064
FINAL GRAVITY: 1.017
FINAL TARGET ABV: 6%
IBUs: 60

[Hokerer]

Reduced hop utilization with concentrated boils comes mainly from the fact that the gravity of the concentrated boil is too high.  You're only changing the amount of the boil from six down to four gallons.  If you put two-thirds of your extract into the four gallons, that'd be the same as if you had put all the extract into the six gallons like the recipe calls for.  Both gravities would be the same so your hop utilization would be too.

[metalhophead]

I would then be topping it off  with probably 1.25 gallons, though, and that would dilute the wort  I'm trying to brew 5 gallons. I assume when I boil the four gallons I will have some amount of boil off. There just has to be a standard procedure for converting a full boil recipe to partial.  Since I"m making an IPA I need to get all of the IBU's somehow.  I assume that you increase the hops a certain percentage. 

[malzig]

I ran a quick recipe using Tinseth's formula and came up with increasing the hops by 20% to account for the volume change if you boil from 4.25 down to 3.25 gallons post-boil volume and add 1.75 gallons in to bring it to 5 gallons.  I think those volumes are more realistic in a 5 gallon kettle, from my experience making 3.25 gallon all-grain batches.

The other concern is the 110 IBU limit of solubility.  This would mean that you couldn't hit 60 IBU if you were only boiling to 2.5 gallons, but since you have the ability to boil 3+ gallons, you dodge that problem.