Ignoring the volume in the tank heads, you can measure the length of the tank and multiply by the following factors, depending on the diameter of the tank:
36″ = 4.4 gals/inch of tank length
48″ = 7.8 gals/in
54″ = 9.9 gals/in
60″ = 12.2 gals/in
72″ = 17.6 gals/in
So a 54″ diameter tank that is 40″ long would hold 396 gallons if completely full, not counting the amount in the head….so you can make a pretty good estimate.