What is the intended IBU of Caribou Slobber?

They can't accurately give you the IBUs unless every time they make a kit they either change the amount of hops based on alpha acid content or change the IBU estimate based on the alpha acid content. In the end, they will still base this on an "average" setup and efficiency. Typically, a recipe is going to call for 1 oz of this hop and 1 oz of that hop and depending on the hop schedule, the alpha acid in each batch of hops, the freshness of the hop and the specifics of your system, the IBU will vary. Most recipes will give a range to account for the slop. In the end, as other's have mentioned, you'll need to do your own calculations using numbers for your system and technique (or use software that does such) to get an accurate estimate, and even then, it will only be an estimate. You can also make some adjustments for the age of the hops on brewday and how they were stored.

Only the recipe designer for Caribou Slobber can tell you what they intended. The IBUs are not listed on NB's website or recipe sheets for extract or all-grain versions. But NB thinly veils (at best) the fact that this is a clone of Big Sky's Moose Drool.

Big Sky claims that the OG of Moose Drool is 13° Plato and it has IBUs of 26. 13° Plato converts to a specific gravity of 1.053, giving Moose Drool a BU/GU of around 0.5. If you believe that a faithful clone is intended then the intended IBU is 26, and the intended OG is around 1.053.

I can also tell you I helped a friend brew the extract version (his first brew) about 14 mos. ago following the recipe exactly in terms of boil volume and hop additions, and did a side-by-side with Moose Drool. The HB version was slightly but preceivably maltier than the commercial original (could be the effect of extract, or underattenuation - my friend put beer on his cellar floor, and then had to unstick fermentation).

It is intended to be a clone. I don't know if the recipe has drifted from being an exact clone to match with hoppier tastes - it was originally designed when brown ales were the craze in the 1990s, and I think I recall it being around back in 1994 when I first started (and quit) HB. The OG of the commercial version and the extract-based clone are substantially the same (1.053 vs. 1.052).

When I plugged the recipe into beersmith, I got 42 IBUs (Tinseth). US Goldings, 5% AA, Liberty, 4.3%, Willamette 5%, full wort boil,

As Denny rightly says, if you're doing a partial boil, then the IBUs will be different:

• 3 gal partial boil gives 28 IBUs
• 4 gal partial boil gives 31 IBUs
• 5 gal partial boil gives 35 IBUs
• 6 gal partial boil gives 37.9 IBUs

(volume is the pre-boil volume going into the kettle. To get to batch size, top up water is added where needed.)

I was puzzled why I get higher IBUs in Beersmith with my setup, and it turns out it's the evaporation rate, plus that I do 90 minute boils (but hops no earlier than 60 mins). A large boil-off makes the wort more dilute initially increases the hop utilization - at least in theory - my tastebuds tell me Beersmith overestimates the hop utilization for my setup.

So if you're building your own recipe around your own equipment (partial boil or large boil-off), you should probably adjust the hop quantities to aim for the 34 IBUs quoted.

If the store told you 34, that's what it is. IBU are based primarily on boil gravity, so if you do a partial boil you will end up with a different figure than if you do a full boil. That could very easily account for some of the differences you see. But people other than those who made the kit are simply calculating their own numbers, through a variety of means that may or may not be accurate. I've designed kits for NB before, so I know the numbers they give you are accurate based on brewing the recipe the way that NB specifies. Using other methods may give you different results.