Very simple computer-aided-design models are introduced to determine the characteristic parameters such as effective permittivities and characteristic impedances of unshielded suspended and inverted microstrip lines. Computer-aided-design models are determined with the use of the genetic programming. The results of computer-aided-design models are compared with the results of quasi-static analysis, experimental works available in the literature and a commercial electromagnetic simulator. The comparison results clearly show that computer-aided-design models proposed in this work are in very good agreement with the simulation, theoretical and experimental results for the suspended and inverted microstrip lines. The design parameter ranges in this work are 2 <= epsilon(r2) <= 20, 0.5 <= w/b <= 10, 0.1 <= a/ b <= 1.5, and the respective characteristic impedances of unshielded suspended and inverted microstrip lines are 28 Omega <= Z(0) <= 185 Omega, and 24 Omega <= Z(0) <= 159 Omega, respectively. It is observed that the accuracies of computer-aided-design models proposed in this paper are good enough for the most practical cases.