By "round container," I presume you mean a right circular cylinder -- the shape of a canned tuna can, but somewhat larger.
The volume of a cylinder is given by the area of the base, i.e. , times the height. Remember that the radius is one-half of the diameter. Divide the volume in cubic inches by 231 to get US liquid gallons, or by 268.8 to get US dry gallons (1/8 of a Winchester bushel) or by 277.4 to get Imperial gallons.
John
My calculator said it, I believe it, that settles it