Recipe Calculations

After the transfer my boil kettle has 1.25 gallons of wort left in the bottom. 

Knowing this...  I make my calculations as follows.

I boil 8 gallons
I loose 1.5 gallons to boil off.
I loose 1.25 gallons to the kettle.
I end up with roughly 5 gallons in my carboy.

I adjust my Beer Smith settings for all this but I continue to miss my target OG.  However, when I go back and change my batch size to 6.25 (My actual post boil volume) I nail the OG.  Should I be setting my batch size for my post boil volume to properly calculate my ingredients?

I set my batch volume to what I have in the kettle at the end of the boil and all of the math works out.  In your case that would be the 6.25 post boil volume.

There is another field in the fermentation section that accepts the total bottling volume...this is your 5 gal in the carboy...or whatever you get out of it at the end.

T-

Actually bottling volume is bottling volume. It is used to figure out the priming sugar.

this seems strange, but i have used 5gallons for my "batch size" meaning that is what i am left with when all is said and done, even though it says that "batch size" refers to the amount going into the fermenter, and my OG's have hit fine.

better question, why are you losing 1.25 gallons to your kettle? can't you simply dump the rest into the carboy?

Bonestar,

It could just be a coincidence that changing the batch size gets you closer to your target OG if you have correctly adjusted for boil off and loss to trub, hops and kettle. Try adjusting the efficiency (of your system) in BeerSmith so that the predicted OG matches the actual OG you got from the brew. Then use the new efficiency % to calculate a new grain bill for the recipe. Keep the batch size at 5 gallons if that's what you expect to yield to the carboy.

Hope this helps,
PC

I just found a nifty efficiency calculator today.

http://www.tastybrew.com/calculators/gravity.html

In your case you'd put your post-boil volume (8 - 1.5 = 6.5) where it says Final Volume.

The batch I did today was 78.5%, the one before 89%(!), before that 66.2%, before that 67.4%, before that 70.3%, before that 82.4%...

Maybe yours is all over the place like mine.