BIAB water amount

I’m going to try my first BIAB soon. Reading various instructions, they call for 6.5 g to 8 g of water for a 5g batch. The idea being the lost water in grain and boil-off will bring the volume down to 5 g.
My thought would be to start at 5g or 5.5g, and add water at the mashout and chill down to equal 5g before yeast pitch.
Seems like it would be less guess work and more accurate.
What am I missing?
Thanks

I treat 7galons and for me that works to get 5.5 in the fermenter. Boil off and grain absorption is the same as other all grain methods. You don’t loose any to tun dead space. If this is your first all grain shoot low you can always top off

I BIAB… I start with 8 gallons… My grist amounts will vary how much water is soaked up and discarded…
What your proposing, mash with a less amount… Add to get the volume you desire…
It’ll work…
Sneezles61

Sure a partial boil works. Many brewers do it for BIAB as well as extract. I prefer a full boil and once you’ve done it a few times and settled on your methodology you’ll know your boil off rate and be able to more accurately determine what you need.

I do a kind of modified 2 vessel BIAB with a sparge. Began using the process when I needed to mash thicker due to a small kettle but kept it up since I get such great results.

I usually brew 10 gal batches but whe I brew 5s I target 5.25 gallons into the fermenter so I target about 6.25 preboil volume. I mash in my BK/MLT with 4.5 gals which generally get’s me around 1.25 qts per pound of grain. I heat another 6 gallons or so of acidified water in my HLT for sparge during the mash. When it’s done i pull the bag and suspend it over the BK, turn the burner on, run a silicone hose in the top of the bag and pump sparge water through the grain bag until I get my boil volume of about 6.25 gals.

Your boil off will depend on how hard you boil as well as some environmental conditions like altitude and humidity in your location. While you’re figuring out the correct volume for your system and process I’d shoot for less since you can always top off in the fermenter. if you reach the end of your boil and your volume is high you could continue to boil so you have a better chance of hitting your target OG but that will affect your hop utilization and bittering as well.

It’s all really trial and error to dial in your specific process. If you’ve been doing extract your should have some idea what your boil off rate is. If you did full volume boils for extract shoot for the same preboil volume.
Just try to be consistent with how hard you boil.

EDIT: The 6 gallons of water I heat for sparge is overkill but I will boil the 2-3 gallons left over then pump it through the system and use it for clean up after the brew day is done.

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I also start with 8 gallons unless I’m making a very light beer.

6 gallons is what I start with in the pot. I sparge with an additional 1 gallon. I have to top off a little at the end after a full boil, in the fermenter, but never very much.

I do my mash indoors and my boil outside so in order to save my back i heat about 4 gallons in 2 electric coffee urns and mash in my kettle. After the mash i pull the grain bag and drop into a bucket. I note the wort level then add whatever water i need to get my 6.5 preboil volume to the grain. I carry the pot out to start the boil. Then pull the bag from the sparge bucket and add that to the kettle. So im not moving more than about 3 gallons at a time. Basically its combo biab with a batch sparge. 7 gallons about

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