We present a fast and accurate closed-form model for interactive computer-aided design and optimization of circular microstrip antennas on suspended or composite substrates. The model facilitates the computation of the resonance frequency, input impedance and bandwidth. We compare the results obtained with the proposed model with those from commercial software (2D-Ensemble and 3D-Microwave Studio), previously published experimental results and previously obtained theoretical results using another full-wave method. The proposed model shows better agreement with experimental results as compared to the commercial two-dimensional and three-dimensional software. Although all theoretical results show reasonable agreement among themselves, the proposed model is at least one thousand times faster than full-wave software.