Concentrated Wort (error in calc) and added water post boil. How to calculate

[Brewer #457861]

I got the ABV figured out from the Dilution and Boiloff Gravity Calc which recommended that I ADD 1/2 Gallon of water. I MEANT to put 3.5 gallons of water in the kettle but mistakenly put in 2.5, leaving me with a scant 1.5-1.75 gal of wort. POST CALC I'm getting the appropriate OG but I have NO clue as to what happens to IBUs. I'd be most appreciative if someone can help me with this BEFORE I waste my batch?! Thanks everyone.

 

[Donoroto]

I got the ABV figured out from the Dilution and Boiloff Gravity Calc which recommended that I ADD 1/2 Gallon of water. I MEANT to put 3.5 gallons of water in the kettle but mistakenly put in 2.5, leaving me with a scant 1.5-1.75 gal of wort. POST CALC I'm getting the appropriate OG but I have NO clue as to what happens to IBUs. I'd be most appreciative if someone can help me with this BEFORE I waste my batch?! Thanks everyone.

IBUs is less a function of water volume than you might expect, but with 30% less water IBUs will be somewhat higher. Depending on the recipe you might not even notice it. Still worth fermenting.

 

[Brewer #457861]

I’m thinking that water is 0 IBU. As long as I have a reasonable IBU calc (which I think I do) it’s a matter of scaling up or down. Maybe I’m over simplifying?

 

[Donoroto]

I’m thinking that water is 0 IBU. As long as I have a reasonable IBU calc (which I think I do) it’s a matter of scaling up or down. Maybe I’m over simplifying?

Water is indeed 0 ibu, but hops are not. That 15g of hops will add ibus, and lower volume means it is more concentrated… until you add water, which dilutes it again.

If you expected to end up with 3 gallons of wort, but ended with 1.75, then added 1.25 (=3) it will be same as you determined in the recipe. Make sense?

 

[Minbari]

Water is indeed 0 ibu, but hops are not. That 15g of hops will add ibus, and lower volume means it is more concentrated… until you add water, which dilutes it again.

If you expected to end up with 3 gallons of wort, but ended with 1.75, then added 1.25 (=3) it will be same as you determined in the recipe. Make sense?

Exactly how it works. I have always purposely added less water in the boil and then topped up to hit gravity.

 

[Zambi]

Exactly how it works. I have always purposely added less water in the boil and then topped up to hit gravity.

I do the same ..

But if I understand the OP correctly:
He has the correct OG, but lower volume.
If so, then diluting will lower the gravity and the alcohol%
(If it was just a calculation exercise, then yes: just add water)