I do not remember the formula

π r^2 for the area of a circle, so π x (12)^2 = 452.4 in^2, area x height=volume so 452.4x48 = 21,714.7 cubic inches, 1 cubic inch = 0.004329 liquid gallon, so 21,714 x.004329 = 94 gallons

And what GeorgeH said. If the tank hasn't been pressurized in a long time you should probably have a hydrostatic test done on it.