On the basis of Onsager's hypothesis a new method is presented to calculate growth rate constants of various crystal faces from the fluctuations of interfaces during NVT simulations. The method is applied to the (100) face of a Lennard-Jones crystal grown from the melt. The results are in perfect agreement with those obtained by means of NPT nonequilibrium simulations. The new method allows for much better statistics at the cost of much less computation time. The use of Onsager's hypothesis to derive the microscopic expression for the growth rate constant may serve as an example for applications in other fields.