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 85" overall length tank holds 504 nominal gallons, when in reality it only holds 460 actual gallons.
The below calculator will give you a true calculation for choosing a proper tank size for your needs.
ID (Inside Diameter) -
Actual Gallons -
Nominal gallons -
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