The estimation of the external quality factor is important for designing coupling devices for the cavities.A new representation of the external quality factor calculations for single-cell cavity coupled to a coaxial transmission line is derived based on analytic analysis and numeric analysis with the help of 3D electromagnetic code,and verified with experimental measurements at room temperature.In logarithmic scale the results for the external quality factor were quasi-linear over the limited range,and the simulated and measured data could be used and extrapolated to the superconducting case.For the unpolished 1.5 GHz 3rd harmonic superconducting cavity,the discrepancy between the evaluation value and measurement result is less than 25% within an acceptable deviation. The estimation of the external quality factor is important for designing coupling devices for the cavities. A new representation of the external quality factor calculations for single-cell cavity coupled to a coaxial transmission line is derived based on analytic analysis and numeric analysis with the help of 3D electromagnetic code, and verified with experimental measurements at room temperature. In logarithmic scale the results for the external quality factor were quasi-linear over the limited range, and the simulated and measured data could be used and extrapolated to the superconducting case. For the unpolished 1.5 GHz 3rd harmonic superconducting cavity, the discrepancy between the evaluation value and measurement result is less than 25% within an acceptable deviation.