Let ABC be a cone. A frustum DECB is cut by a plane parallel to its base. Let r_{1} and r_{2} be the radii of the ends of the frustum of the cone and h be the height of the frustum of the cone.

In ΔABG and ΔADF, DF||BG

∴ ΔABG ∼ ΔADF

CSA of frustum DECB = CSA of cone ABC − CSA cone ADE

CSA of frustum = π(r_{1}+r_{2})l

Total surface area of frustum

=CSA of frustum + Area of upper circular end + Area of lower circular end