I am just an E-5, but the manufacturer is required to label the tank where the label can be seen. Again, an example 42" x 24" x 12" =12,096 now divide by 231 and you get approximately 52.37 gallons.
Can you measure the length, width, and height of the tank??
If so, multiply those numbers (eg. 42x24x20) then divide by 231. That will give you the number of gallons.
