If it's a bucket then 5 gallons is probably a sensible maximum.
If it's one of those â5 gallonâ closed fermenters with a 4" screw top, they actually hold 30 litres (6.66 gallons) and after the âwhooshâ has subsided you can fill right up to the neck. Makes quite a lump if you've later got to lift it from floor to worktop though.
Got 2 of those on the go now with 6 gals in each, looks as though they have room for 1 1/2 gal more each, dont know why they are called 5 gal f v's, Dave NE