A cost optimization technique for a complete design of singly reinforced rectangular beams is presented. The procedure is formulated to find the design parameters which produce the minimum cost of designed section with constraints imposed based on ACI code provisions. The cost of concrete, flexural reinforcement and formwork are included in the formulation of the objective function. The behavioral constraints, i.e, ACI requirements for flexure and shear strength serviceability requirements, i.e., deflection and practical and architectural constraints, i.e, limitation on cross-section dimensions of beams are included in the formulation. A closed form solution of the optimization problem expresses the optimal values of the flexure reinforcement, shear reinforcement and section dimensions in terms of material costs, strength and ductility parameters. A complete flow chart and a computer program of the algorithm is presented and several numerical examples are given.