Recipe Numbers Not Adding Up: 5 Common Causes

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Ever find a great looking recipe - on a website, in a book, or elsewhere plug it into your brewing software - and the numbers just don't add up?
I'm specifically talking about the IBUs and the specific gravity.
It can be frustrating!
Here are the main reasons why that happens:
(Note: I've included screenshots from BeerSmith on where to look for these numbers. Any good brewing software will have these, however.)
1. Efficiency
Proper homebrew etiquette dictates that when you publish your recipe, you state your assumed efficiency. But a lot of brewers got bad manners!
Efficiency is the first place to look.
If you plug a recipe into your brewing software verbatim, and your gravity doesn't match up, it's likely because they assumed a different efficiency.
Of course, you can figure out the efficiency. You would need to enter their recipe, observe the original gravity, and tweak the efficiency until you hit their stated original gravity.
But that sure is a pain. So I'll say it again - for the good of homebrewers everywhere, state your efficiency!

Mash Efficiency
2. Extract Potential
This one is more elusive. Before I knew to look for this, I fumbled for hours trying to figure out why my numbers didn't match up.
My efficiency was correct. My volumes were spot on. My grain amounts matched exactly.
What was the problem? The extract potential in my brewing software was different than what the author assumed.
Just one point different, but it was enough to cause a discrepancy in the OG. The extract potential refers to fermentable sugar contributed by the grain.
We sometimes forget that in brewing we're dealing with living (or formerly living) things. Hops, malt, and yeast are not artificial ingredients made in a lab. That means they don't always behave predictably. Malt composition changes.
It's helpful to learn how to read a malt analysis sheet. Keep in mind that the extract is in reference to pure sugar (sucrose) which has a ppg (points/pound/gallon) of 46. The ppg is the number you want to focus on because it's the one you'll tweak in your brewing software.
I recommend getting familiar with certain types of grains. If you buy from a local homebrew store, use the same brand over and over again. Eventually you'll learn how it performs, not just in taste but in its extract yield.
This is an area where the art and science of homebrewing merge fun stuff.

Grain Potential
3. Alpha Acids
Alpha acids chemicals that contribute to the bitterness of the beer. Hops vary greatly in their alpha acid content depending on their variety. From the 3% A.A. Saaz hops to the 16% A.A. ultra potent Warrior hops.
But don't be fooled!
Just like the extract potential of malt changes, so does the alpha acid content of hops. Even within a single variety.
The default value for a particular hop in your brewing software is just that - a default value. Always edit that number to what is printed on the label of your hop package. That will ensure your IBUs match up.

Alpha Acid
4. Hop Formula
If your hop amounts, alpha acids, and volumes are correct and you're still not getting the right IBUs, then there's a good chance that you're using a different hop formula than the recipe author.
There are three main hop formulas: Tinseth, Raeger, and Gaeretz. Each will give you a slightly different IBU figure.
Most brewing software lets you easily switch between the formulas, so if your number is off, trying toggling through the formulas and see if you can get it to match up.
Personally, I use Tinseth and stick with that. There is no "correct" formula. It's more important to just pick one and be consistent in all your recipes.

5. Liquid volumes, especially "batch size"
Unfortunately there is no agreed upon definition of "batch size".
A few of the ways I've seen defined:
  • Volume at flame out
  • Volume into the fermentor
  • Volume into bottles/kegs
My favorite way is: Volume at flameout.
The reason is because this is where the composition of the wort gets locked in. Once you're done boiling, the gravity of the wort and the IBUs do not change.
As we know, boiling increases both the IBUs and gravity.
After the boil everyone has their own techniques: hopping, recirculating, whirlpooling, etc. If we all called "batch size" the volume at flameout it would cause less confusion.

Batch Volume
So there you have it. A few of the main reasons why your numbers may different from those in a published recipe. If I've saved someone from even a few minutes of frustration, I've done my job.
The BeerSmith "Batch Volume" definition is volume to fermenter (ie. "Post Boil Volume" - "Loss to Trub and Chiller") which differs from what you prefer. Look at the numbers in your screen shot. I suggest you not push to redefine what has been in use in a very common brewing software. I'm not sure what the convention is in other software, as I use BeerSmith. Also, why did you zero out "Cooling Shrinkage"? This will lead to errors in the calculations.
Other than that, the information in this article should help a lot of people who haven't previously understood how all of this works.
I agree with the other Doug. The BeerSmith batch volume is what goes into the fermenter; it does not include the trub that gets left behind. It's how BeerSmith calculates your total efficiency (i.e., "Brewhouse Efficiency").
Also, I noticed that mash tun weight and specific heat were both set to zero in your screen shot. That is going to throw off the calculation for your strike water temperature.
As an aside to the other Doug, I set my BeerSmith mash tun numbers to zero as I warm up strike water in my BKettle and mash in it, pseudo BIAB, making my tun already at temp with strike, so maybe that's the situation
To the questions about BeerSmith, yes, I call "batch volume" the volume at flameout while BS uses it to mean the volume into the fermenter. I am not redefining the term. Brewing Classic Styles, the recipe bible, uses 6 gallon batches. In fact, many of the recipes you find posted are stating their post-boil volume when they say "batch size."
This discussion in the comments reinforces the article's point of why a brewer's numbers may not add up. We're defining things differently! That's why it's important to exercise caution when copying a recipe into your own software.
Great write up, I had many of the same issues when I started using BS, I still have issues from time to time with IBUs not matching exactly, will do more research on tinseth etc, thanks for posting this!
Some good points here, even if we don't all choose to do things the same way. Like Billy, I chose to use batch volume as at the point of flame out in my process using Beersmith. This is because I found that different beers produce different batch sizes as defined by the program (volume into fermenter) depending on ingredients and process.
For example, an IPA may result in a much larger loss to hops and trub as well as the hoses in my whirlpool setup compared to a medium-alcohol beer with an ounce and a bit of hops and no whirlpool. This is of greater importance since Beersmith defines efficiency based on the batch volume and it was driving me nuts changing all the variables to reflect what happened (yes, I'm a bit OCD...). By defining batch volume as at flame-out, I can leave my efficiency at 71% and the variability of volume after that is ignored in my recipes.
I use Brewing Classic Styles and am JUST learning BeerSmith. Do I simply make the change in BS to show 6.0 gallons rather than 5.50gallons? If so, does that affect anything else in the software that I need to know about? Also, OCD'ing. Is this 6gallons.. or whatever the number is.. measured cold or hot? I guess it could be cold as after the chill and before transferring.
Thanks, I have a LOT to learn re BeerSmith