The volume of a cone is always 3 times smaller than that of its corresponding cylinder (that is, a cylinder with the same height and base radius).
As in:
Volume of a cylinder= πr²h
and Volume of a cone=⅓πr²h
But just in case you were referring to an actual difference, then that would be ⅔πr²h, since 1—⅓ = ⅔.