Easy newb question based in a recipe I'm reading

6.3 gallons batch size implies 6.3 gallons of beer going into the fermenter, which would need MORE than 6.3 gallons of water to start. You have to account for hop absorption, any dead spaces in your system, and boil-off (and grain absorption in all-grain or partial mash brewing). Boil off is usually at least a gallon for a typical full boil, but often more. Hop absorption varies by how much hops you add, and some systems have large dead spaces, while some may not have any dead space.

 

The 8.25 is preboil, 6.3 is after boiloff and trub loss. Batch size = the amount you are fermenting