Drainage from an aqueous foam plays a pivotal role in determining its stability. A simple and convenient way to study drainage is to monitor with time the volume of liquid draining from a standing foam. The distribution of liquid in the foam at the start of the experiment, i.e., when foam of the required height is formed, depends on the method used to generate the foam and strongly influences the drainage process. An unsteady state model is proposed for computing the distribution of liquid in a foam column during its generation by bubbling. This model accounts for the propagation of the foam front during foam formation. The distribution of liquid in the formed foam column differs significantly from that obtained using a quasi-steady state approach, indicating that the inherently unsteady nature of foam formation cannot be ignored. The drainage curves obtained using the present model show good agreement with experiment.