don't know, but if you really need to know weigh it empty, fill it with water, weigh again, take the difference and divide that by 8.35. that'll give you volume in gallons.
I have the 6 gallon one and i made gallon marks on it using a measuring cup. 6 gallong was halfway between the shoulder and the bottom of the neck. So probably 6.25 gallons would overflow it i would say. Should be about the same for the 5 gallon one i would guess.