Jeff's question can be answered from knowing the inside dimensions of any tank - length and diameter in this case.
Those dimensions would be an few inches LESS than the outside measurements.
Calculate the cubic footage of the tank interior as a cylinder. Do not include dome capacity.
100 gallons occupies13.36 cubic feet, with a water weight of 213.76 lbs.
(1 gallon is 1.336 cubic feet. A gallon of water has 16 pints weighing a pound each, if you are seeking weight).
Divide the total volume of the tank by 13.36 to get the gallonage in 100's.
Round this down to the nearest gross thousands to find a nominal gallon capacity.
These are often stated as 5,000, 6,000, 8,000,10,000 12,000 etc. but could vary by being slightly larger as well.
For water weight capacity in pounds of each nominal size tank, multiply by 16.
I think water weight for fluid loads was generally used in weight capacity calculations.
These determined journal / bearing sizes for the trucks used under a tank car.
Other fluids may vary by weight, being lighter or heavier from their chemical composition.
A gallon of gasoline weighs less than a gallon of water, for example.
A gallon of sulfuric acid is heavier.