The simple EOQ lot sizing model or some variant of it is often used in discrete parts production lot sizing to obtain ballpark estimates of economic production quantities. Such formulas ignore the influence of lot sizing on work-in-process inventory. In group technology production there is a traceable relationship between lot size and work-in-progess inventory. This paper explores this relationship and describes an economic lot sizing model appropriate to group technology. Experience with the model indicates that its use can lead to somewhat different results from those which would be obtained using the standard EOQ formulation.