The Truth On Proper Tank Sizes And Actual Gallons Vs.
Nominal Gallons
Storage tank manufacturers may use either actual or nominal gallon capacities.
Nominal gallons normally include the space on the elliptical heads as storage
area (white area in image below).
For example, a 42" outside diameter x 84" overall length tank holds
approximately 504 nominal gallons, when in reality it only holds approximately
456 actual gallons.
The below calculator will give you a close approximation calculation for
choosing a proper tank size for your needs.