How can I calculate what IBU I need in the wort if I top up with water?

Elysium said:

I know that one of the important things will be hop utilization factor but that is the easy part.

How do I know what IBU the wort should have if it gets diluted and I want to reach X IBU in the final product?

Say I brew a 2 gallon batch and top it up with 1 gallon of water....the IBU should be at 40 after the fermentation, but what is the IBU in the 2 gallon wort before dilution?

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Yooper said:

You can calculate the IBUs with brewing software, unless you have a way to have them testing.

Say your software says you have 50 IBUs in 2.5 gallons. And you add 2.5 gallons of water (0) IBUs. You've reduced the IBUs by 50%, via dilution, for 25 IBUs.

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