Batch Size Conversion Question

So, looking at a recipe.. let's take BM's Centennial Blond as a for instance.

Batch Size is 5.5 gal; Boil Size is 6.57 gal.

I'm assuming that the recipe is figuring on bottling 5 gals with about 0.5 gals left in the fermenter as trub, etc.

That said.. If I want to take that recipe and make a 3 gal (2.5+ g to the keg) conversion and figure 3.7+ gals as my Boil Size....

Do I modify the ingredients so that they are cut in half? IOW, IF that recipe is really for 5 gals irrespective of boil off and trub, am I correct in figuring that mine will be 2.5 gals irrespective of what the boil size and batch size is?

Hope I expressed this clearly..

you can cut your ingredients approximately in half(but not exactly in half), but you need to figure out your own boil off rate based on your pot

i brew one gallon batches... i compute my ingredients based upon the volume going into the fermenter (1 gallon), not based upon the volume going into the boil pot, or going into the bottle.

i know that based upon my brewpot, and the settings on my stove, how much boiloff i get in an hour.

i know, also, that i am going to lose about a quart of my output to trub, etc. but since i ferment in a glass carboy, that's just the way it is.

so
1. decide on the BATCH size, and scale your ingredients to that.
2. adjust your BOIL size, to hit the Batch size, based on your equipment.
3. live with the trub loss.

hope that helps

Thanks.. My question is/was.. is the original recipe based on the boil size or what you think will be going into the keg. Ie.. 5.5 gal's vs what I'd guess would be 5 +/- gals.. maybe I'm being too anal..

it should be what is going into the fermentor. You will lose some more to trub and transfer loss when going to the keg. So 5.5 gallons into the fermentor will probably be about 5 gallons going into the keg or bottles.

i'm about 90% sure most recipes are based on the batch size... but there's a lot smarter people in here than me to answer that.

the batch size is what goes into the FERMENTER, not the keg, and not the beginning of the boil.

it's what you have when you get finished with your boil.

go back to my example, because the math is easy. my carboy holds exactly one gallon, so my batch size is a gallon.

i put water into the pot based on the length of the boil (usually 60 minutes), to insure that i have just a touch over one gallon left, so my boil size it about 1 gallon, 1 quart, and one cup.

when i bottle, i get 8 - 12 oz bottles (96 oz), so I lose a quart to trub.

i scale my ingredients to the one gallon BATCH size.

and that isn't too anal. it's pretty important.

hope that makes sense, cuz it's the best i can do

Yeah, batch size is the amount going into the fermentor.

Scaling recipes is a little more in involved that just dividing by X. There is math involved in order to maintain the bitterness ratio, etc. The easiest way to do it is download Brewsmith (free trial download) and hit the "scale" button.

As for boil-off, the boil-off will be the same for 5 gallons as it is for 3 gallons... it's a function of surface area (your equipment profile) and environment (more will boil off in dry climates that moist).