I aske because I have a 6.5 gallon carboy that can hold almost 7.5 gallons of water. And my 5 gallon carboy can only hold about 5.25 gallons of water. DOes this make any sense?
I think the 6.5 gallon carboy holds 6.5 gallons at the curve. Same with the 5 gallon, etc. So, it would hold more if you count the top of the curve to the next.
same reason my shoe size is 10.5 for sneakers, but 11 for boots... manufacturer variations.
i have 6 gallon carboys that are 6 gallons up to the very top, and i have 5 gallon carboys that are 5.25 gallons up to the shoulder. just depends on where you measure from. everyone does it differently.