Water absorption is roughly .2-.25 gallons per pound of grain.
Water weighs about 8lbs per gallon.
4 lbs of wet grain will weigh 10.5-12lbs.
These are all ballpark numbers that are close enough for me.
Thank you. The water absorption is what I didn't have. Though I just thought to check if Beersmith had BIAB equipment profiles and it does, so I could have backed into an answer through the recipe. It's still really good to have this going in. Triangulation, you know, all in the pursuit of... homebrew!
A 2.5-gallon batch using 5.5 lbs of grain: at .25 gallon absorption, that would add 1.375 gallons, or 11 pounds. That's a 16.5 lb bag. Practicing with a 20-lb bag of sugar or flour ought to give me a sense of how easy it is to hoist a bag out of the kettle.
The Beersmith "mini" BIAB profile is based on a 5-gallon kettle and a 2.91 gallon batch. But that would be almost 4 gallons of water, according to Beersmith. It sounds as if it might be wiser to use my 8-gallon kettle.