The design of rigid pavements according to AUSTROADS, is a lengthy method. The designer assumes a pavement structure then uses a number of tables and figures to calculate the two governing design criteria, the flexural fatigue of the concrete base and the erosion of the subgrade/sub-base. Each of these two criteria needs to be less than 100%. Ideally, they need to be at their maximum possible value that is less than 100%. Designers would repeat the design if either of the criteria is more than 100%, in other words unsafe design. However if the criteria are much less than 100%, ie over-design, most designers would stop after one or two iterations due to the lengthy process involved in calculating the criteria, and due to time limitations. This leads to designs that are safe, but not necessarily economical. This paper presents a formulation for the problem of optimum design of rigid road pavements by defining the objective function, which is the total cost of pavement materials, and all the constraints that influence the design. All these are given in terms of design variables and design parameters. The formulation including the optimizer was implemented in a spreadsheet. The optimization problem is used in a parametric study where 1680 pavement structures are optimally designed.