How do you guys calculate the IBU when you use top-off water after the boil?

So....I have come across this idea to use top-off water, which would be great since my pot is not that big.

However.....there is this little problem I am facing: how do I calculate the final IBU of my beer?

I am pretty sure some of you guys also use some top-off water at the end of the boil and have means to calculate the final IBU. Can anyone point me into the right direction?

For example...if I brew a 6 gallon batch of AAA at 1.070 at 35 IBU and I top it up with 1.92 gallons of water to get an 8 gallon batch at 1.053...then what happens to the IBU? How will it be affected by the top-off water?

 

Take the fractional value of wort you started with and divide by total volume and reduce the IBU by the same amount.

6 gallon / 8 gallon = .75 so 35 IBU / .75 = 26.25 IBU

 

Ok, thanks.

Shall I do the same with aroma hops too? I mean the ones that dont contribute IBU at all...cause I add them after the flame-off.

An 8 gallon batch is 1.33 times bigger than a 6-gallon batch...does that mean that if I normally put 1 oz in a 6 gallon batch...then I will need 1.33 oz to get the same result with the diluted 8 gallon batch? Is it that simple or is there anything else to account for when it comes to alpha acids, aroma or flavour?

 

technically you need a little bit extra of bittering to get the same as you will be boiling a more concentrated wort and thus get worse utilization, but ratio should be close enough.

 

Yeah. Palmer talks about hop utilization. I think I look into that chapter again and see if I need to adjust the amount hops even further.

Thanks.