Would be the max grain bill in a 5 gallon cooler w/say 4.5 gal water?
BIAB won't impact the calculations from the linked site.
Just using the site, quickly, you should be able to safely mash 12# of grain at 1.25qt/# (water to grain ratio) in a 5 gallon vessel/pot/cooler/etc.
Personally, I wouldn't go with a 5 gallon cooler mash tun, unless you're making 3 gallon batches. IMO, you're going to be very limited in what you'll be able to brew, at least in all grain. If you're going partial mash, then it could work out.
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