Top off water impact on IBU?

So adding some equipment and using a different process i overshot my OG by 5 points and fell short on volume. I calculate I can add .5 gallons of boiled water to hit my target OG. What impact would this have on IBU? I brewed a munich helles targeting 1.049 and 17 IBU. I have 4.5 gallons of 1.054 wort chilling in the fridge prior to pitching.

Thanks

It would be reduced by whatever percentage .5 gallons is of your whole batch. So .5 is about 11% of 4.5, so if your 4.5 gallons are 17 IBU it will be reduced to about 15 IBU by adding .5 gallons plain water

Thanks, perhaps I'll just call it a day then!

If the recipe was designed to yield 17 IBUs in 5 gallons, if you top up to 5 gallons then that is what you will get.

At 4.5 gallons, you will have about 10% more IBUs, or somewhere around 19 IBUs.

Of course the actual IBUs is just a guess, so you don't really know what you have.

Bottom line, if you make the volume up to 5 gallons you will get your intended bitterness, if you leave it at 4.5 gallons, you will have 10% more.

Calder said:

If the recipe was designed to yield 17 IBUs in 5 gallons, if you top up to 5 gallons then that is what you will get.



At 4.5 gallons, you will have about 10% more IBUs, or somewhere around 19 IBUs.



Of course the actual IBUs is just a guess, so you don't really know what you have.



Bottom line, if you make the volume up to 5 gallons you will get your intended bitterness, if you leave it at 4.5 gallons, you will have 10% more.

Click to expand...

Thanks, I am trying to map out how this happened so I can get a better estimate. It seems my boil off rate increased with a better burner and I kept the trub in the pot and underestimated that amount. I think that is how I fell short .75 gallons. I also think my new grain mill added to the OG, so I will compensate next time. If it's true the IBU increased a bit, then it should be fine. It's just me drinking it anyway, so .5 gallons isn't a big deal. Probably not worth the hassle after 4.5 hours of brewing.

Slightly over-boiling is no big deal actually, it happens to a lot of brewers until they get their equipment dialed in. IMHO having slightly less than my initial targeted volume-meaning I need to top up, is a lot better than under boiling and and not hitting my desired O.G. by being too dilute.