The I.D. of 12" NPS x 0.219" wal pipe would be 12.75" - (2 x 0.219") = 12.312"
The volume of 1 ft. (12") would be pi x (12.312" / 2)squared x 12" = 1428.68 cu. in.
1 US gallon = 231.000001 cubic inches, so the number of gallons = 1428.68 / 231.000001 = 6.185 U.S. gallons.