IBU Calculations - missing something

[jacketsdb23]

I'm on my second beer and 1st time attempting a porter. I wanted to try to calculate my IBU this time using the method in John Palmer's book.

So:

IBU = AAU*U*75/V

Porter recipe = 1 oz 6% AA (6 AAU) Boiled for 60 minutes
1 oz 6% AA (6 AAU) Boiled for 1 minute

The original gravity (OG) standardized value is 1.054

I estimated a U of .221 for the 60 minute hop boil
I estimated a U of .008 for the 1 minute hop boil

Total IBU = 21.6? The recipe says it should have an IBU of 33.5. Can anyone see what I'm not doing right?

Thanks!

Marcel

 

[devaspawn]

Could you give us the rest of the ingredients?

 

[jacketsdb23]

The Malt Extracts were 7lbs ultralight and 4 oz of Maltodextrin

Steeping Grains are 1lb crystal 40L, 8oz Black Patent, 4 oz chocolate

sorry, new to this

 

[TeleTwanger]

Yeah I'm getting 18 (1X6X20/6.7)17.9

I wouldn't even add anything for a 1 minute boil but if i did it would be 1X6X1/6.7=.89(1) For a grand total of 19IBUs.

Seems like he's giving 30%U for 60 min and 8%U for 1 minute ....I give 25%U for AG @ 60 and 20%U for PM @ 60. and 0% U @ less than 10 min.

 

[devaspawn]

What will your pre-boil volume be?

 

[jacketsdb23]

What will your pre-boil volume be?

Pre-boil was 6 gallons