The formula
For a circular cone, V = πr²h/3, where r is the radius of the base and h is the perpendicular height.
An example
With radius 3 cm and height 8 cm, V = π × 9 × 8/3 = 24π cm³, approximately 75.40 cm³.
Use the height, not the slant length
The slant length follows the side of the cone. The height goes perpendicularly from the base plane to the tip. Substituting the slant length overstates the volume.