How much volume does a cylindrical tank hold?
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