The volume of every sphere is
Volume = (4/3) (pi) (radius)³
When you say "across", I think you mean the diameter of the ball.
The radius is half of the diameter = 12 inches.
Volume = (4/3) (pi) (12 inches)³
= (4/3) (pi) (1,728 cubic inches)
= 7,238.2 cubic inches . (rounded)