Lylo, you always ask the best questions

. This is a really good question

.
I can't answer it and I'm not sure if anyone really can.
This whole area is so jumbled at so many levels it's way beyond ridiculous. At the highest level, let's assume that you own the scientific instruments that can accurately measure IBU's. Problem 1: (I'm putting this in brackets as I need to check something here but I'm almost 100% sure that there are several different methods of how IBU's can be 'scientifically' measured and that there are many problems involved in when and where the sample is collected.)
Let's assume though that I have the above totally wrong and that any beer's IBU's can be correctly and accurately measured. Is this what the BJCP is basing their IBU definitions on? They don't say but let's assume they are...
Let's pretend an IBU is an IBU
So. let's say that there actually
is an agreed scientific way of measuring IBU's and that the BJCP has followed this method. In other words, this particular lager recipe
will give you 20 IBU's.
Now, put this exact recipe into any program or calculator and see if you can come up with 20 IBU's. You won't

.
Atm, we have three existing formulas used in calculators/programs to estimate the probably fictitious 20 'scientific' IBU's
Look at the pic in
this post. Every estimated IBU is different for exactly the same recipe depending on which bitterness estimate formula has been used.
Over and above this, the software writers have often interpreted the above formulas incorrectly.
One of the most respected software programs around has made many mistakes in just one of their hop formulas and some of these are still present. It took years for a few of us to even get our heads around, "How could they get such a simple formula wrong?" Luckily we all came together in a previous thread here and were scratching our heads at the same time. The existing 'The Calculator' and the upcoming 'The BIABacus' formulas are correct but only if we have assumed that Tinseth, by batch size, meant the end of boil volume at ambient. (I'd have to look through a lot of emails but I'm pretty sure he said yes to this one.)
What were you expecting Lylo?
Lylo, like me and any other sane brewer, you would expect that an IBU is an IBU and that formulas and programs would estimate them correctly.
They don't. They are faulted at many levels.
The best base we have to go on for all-grain atm is the Tinseth formula interpreted at a common sense level. (Would have to check my emails but I think even Tinseth agrees with our interpretation of his 'batch size'. If he doesn't then the formula would be useless anyway.)
So, it's a hard thing to get your head around IBU's
I really hope the above has confused you Lylo as it will make you feel more confident. This whole area is faulted yet few people question it. So mate, that is a real credit to you

.
Should we worry about it?... I don't think so but, it is very nice to see the right questions being asked.
Good stuff Lyle

,
PP
P.S. When you study this whole area of IBU estimation, you'd be shocked at how little research these formulas have been based on. Have a look at
this TED talk. By the end of watching that, you should be confident you are asking the right questions

. The real point of the talk is about the consequences of when people do not ask the right questions. If we can't ask the right questions in home brewing then may the brewing gods help us on the really important stuff

.
P.P.S. I'm away for a week and a bit so please excuse any delay in reply

.
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