Calculating IBUs

I know how to calculate IBUs no problem. My question is what do you do if your SG changes significantly during your boil. For example, say I only add a little extract, or have a low SG run off from a partial mash or a steep and add my bittering hops. After say 30 mins, I add the remaining extract and now have a high SG boil for an additional 30 mins. For IBU calcs do I consider a 30 min boil at the initial SG and a seperate 30 min boil at the higher SG? Hopefully someone sees where I'm coming from and can give me an idea if they know. The reason I'm looking into something like this is b/c I want to make an Excel spreadsheet with some simple calculations (IBUs, Gravity Units needed to add/remove to make a target OG, etc).

If it is true, that you do want to spilt up the IBU calcs by SG of boil, is there another issue?--->Being does it matter that a hop pellet has boiled for 30 mins already when considering the additional 30 min at another SG? I don't know if a lot of oils have been extracted already and it wouldn't be fair to count those hops at the higher SG boil at their initial AAU value?


Any people down with brewing math/science please chime in!

Thats a tough one since a percentage of the oils have already been extracted when your SG changes so treated as a new calc and a new SG isn't accurate.

How much do you estimate your SG will change? A quick run on my spreadsheet shows that there isn't a significant difference. A 60min boil of 6AAU only changes the IBU -2.29 when the SG is adjusted from 1.050 to 1.040.

After looking at the equation I use a little closer it assumes a constant rate of extaction of the oils. Meaning it doesn't account for more of the oils being extracted early in the boil process rather than later. Since this is the case you could simply give the SG a weighted average of how much time it spent at each SG. (Sounds confusing the way I'm saying it)

EX: 1.040 for 20 min and 1.050 for 40 min would be
(1.040 x .333) + (1.050 x .667) = 1.04667 SG

jcarson83 said:

After looking at the equation I use a little closer it assumes a constant rate of extaction of the oils. Meaning it doesn't account for more of the oils being extracted early in the boil process rather than later. Since this is the case you could simply give the SG a weighted average of how much time it spent at each SG. (Sounds confusing the way I'm saying it)

EX: 1.040 for 20 min and 1.050 for 40 min would be
(1.040 x .333) + (1.050 x .667) = 1.04667 SG

Click to expand...

That seems logical to me. I think I'll want to account for the change in SG especially since I do partial boils with adding the extract late. So for example begin with a low SG ~1.030-1.040 (from a steep or small PM) boil and then add all the extract and boil for the remaining time. I believe SG will probably jump into the 1.070-1.090 range for a partial boil (3.5-3.75 gallon). I will try that method out. Thanks!

I've never heard of adding extract for a partial boil. What does this do for you?

jcarson83 said:

I've never heard of adding extract for a partial boil. What does this do for you?

Click to expand...

I wasn't very clear, sorry about that. I have water that I steep grains in or in other cases a mini-mash I do in the oven. I take that water (now wort) bring up to ~3.5gallons and begin the boil. I add bittering hops and begin my 60 min timing (at low SG). I add the extract for the recipe late (maybe with only 20-30 mins remaining on the boil, I do this b/c it keeps color lighter) so when I add the extract into my boil the SG will rise significantly and hop utilization will become MUCH lower. After the boil is done I'll cool the wort, bring up to 5.25 gallons in my fermentor and go from there. Does that help clarify?

I just figured the IBUs on the recipator were not very accurate if I'm adding my extract late in the boil instead of the entire 60 min.

jcarson83 said:

EX: 1.040 for 20 min and 1.050 for 40 min would be
(1.040 x .333) + (1.050 x .667) = 1.04667 SG

Click to expand...

I decided to go ahead with the weighted average SG for my boil outlined above.

For a hypothetical boil (rough estimate of a recipe I'm working on) 1st 30mins of the 60 min boil will be 1.035 and the 2nd half of the boil will be 1.088. The difference in IBUs doing a weighted average and the entire boil at SG 1.088 is only 3.5 IBUs, I guess a lot about nothing. The important thing is my spreadsheet math is correct and I'm like using it, I'd send a copy if you want to see what the hell I'm blabbing about.

Yeah I'd like to see it, I'll send you a copy of mine too.

jcarson83 at gmail.com

Cool, I'll be sending it now, my address is:
jmsieglaff at yahoo dot com

You don't get a constant rate of utilization. A typical schedule would look like:

5% @ five minutes
10% @ ten
15% @ 20
20% @ 35
25% @ 60
27% @ 120

Thanks. It sounded good though.

david_42 said:

You don't get a constant rate of utilization. A typical schedule would look like:

5% @ five minutes
10% @ ten
15% @ 20
20% @ 35
25% @ 60
27% @ 120

Click to expand...

Well, I first want to say... I have no idea why hops confuse me. I keep reading different material to try to figure out how much hops I want and all... but tend to fall back on a recipe example to guidance.

One thing I wanted to be sure of though David, this why you add aroma hops for only a few minutes.... because of the rate of utilization. Right?