The freezing point depression can be calculated using this formula:
ΔTf = i * Kf * m
Where ΔTf is the change in the freezing point, i is the Vant Hoffs factor, Kf is the freezing point depression constant and m is the molality of the solution.
The Kf (molal freezing point depression constant) is defined as the change in the freezing point for a 1 molal solution of a nonionizing solute. In other words is the change in the freezing point when i and m are 1.