This dissertation presents two inventory models applied to the continuous review, order-point, order-quantity (Q, R), that define a fixed quantity to be ordered and the occasion to order. The objective is to minimize the total annual cost comprised by setup, backorder/stockout and holding costs. First, these models are tested with their complete formulation and compared with approximations proposed by many authors. Second, it is developed a formulation to facilitate the identification of real cases where approximation is recommended. Finally one of these models is applied to define optimum inventory levels of oil products in a Petrobras` refinery and results are compared with the level of inventory the company now holds.