Ibu math

[sictransit701]

Does anyone have the math to figure ibu’s?
I’m using brewsmith. It gives me ibu’s based on my estimated pre boil gravity, but what if I don’t reach that gravity? I would need to adjust my hop amounts. Trying to figure that out

 

[Brewer393052]

Are you referring to the math used in that kind of calculator?
https://www.brewersfriend.com/ibu-calculator/

 

[Climb]

If you don’t reach your pre boil gravity target, you have several options:
1. you can add DME to reach your pregravity target
2. boil for a bit to increase the gravity before you start your boil timer
3. or as you are mentioning, decrease your hops additions to decrease the total IBU in the recipe.
I assume you would adjust the IBUs while maintaining the original recipe’s BU:GU ratio. But what hop addition are you going to reduce from?
Can’t you just adjust things in BeerSmith and let it’s IBU calculations help guide you to your new hop additions? Save your original recipe as a new recipe. Decrease your malt bill to match your measured pre boil gravity. Adjust your hops as desired.

 

[hotbeer]

Adjust the stuff in your recipe software to match the SG's to what you are getting in real life. Then you shouldn't have the disparity in your pre-boil SG and theirs.

 

[StevoTosh]

This what I use in my brew calculator on Excel.
The table plots points on the graph. I find it is easier for the calculator to find these values based upon the boil gravity (calculated), and the quantity, AAU and time (entered), rather than ( more accurately) finding points on the graph of the equations which are written at the bottom of the table.

This is the calculation I use:

IBU = (Quantity of hops(g) x AAU(%) x utilisation value(from table) x 10) / post boil volume of wort(L)

 

[StevoTosh]

This may be an easier way to look at it.

Your adjusted hop amount ( in grams) is equal to:

( [desired IBUs] x [post-boil volume in Litres] ) all divided by ([AAU%] x [utilisation value from table] x 10)

Edit: this method has been very accurate for me when using pellet hops.