You probably know the formulas for various prisms. All of these formulas use a basic formulation V=Bh where B is the cross-sectional area of the parallel bases and h the perpendicular distance between bases.
(1) parallelpipeds ("boxes") V=(lxw)h
(2) triangular prisms V=(1/2bh)H where H is the perpendicular distance between the parallel bases and h is the height of the triangular base.
(3) prisms with regular polygons as bases V=(1/2ap)h where a is the apothem and p the perimeter of a base.
(4) cylinders `V=(pir^2)h` where r is the radius of the base.
In each case the cross-sectional area is the same as the area of a base and the formula is V=Bh
This can be extended to figures of any shape whose cross-sectional area remains constant. To find teh volume of a loaf of bread, if we assume each slice has the same area, is just the area of a slice times the length of the loaf. Finding the area of the slice may require advanced techniques, but the result still holds.