Water Heater Capacity

A gallon is 231 cubic inches. A cylinder 2 inches long and a hair less than 24-1/4 inches in diameter holds 2 gallons. (assuming I did the math right, of course -- looks about right to my eyes.)

It won't hold 10 gallons 'actual' but 10 gallons 'nominal'

I have always understood that the heater capacity is the number quoted. Often the fresh water capacity is stated as the size of the fresh water tank plus the size of the water heater.

Please let us know after you take cpaulsen's advice.

Just fill that 10 gallon tank you pulled and see how much it holds.