Free-space methods for EMC antenna calibrations at Open Area Test Site (OATS) are not suitable for monopole antennas because of corresponding large wavelengths. The Equivalent Capacitor Substitution Method (ECSM) is adopted in ANSI and CISPR standards. One of major uncertainty factors of ECSM is the accuracy of capacitance. A novel capacitance extraction method based on full-wave impedance modelling is proposed in this paper, which is more accurate than conventional analytical formulae and experienced values, especially for practical monopole antennas, with a telescopic rod, or with a finite ground plane. A projection based model order reduction method is then utilized to achieve a compact macro-model. The capacitance value can be determined from the input impedance which is given by the macro-model output. The scheme and computational methodology of a developed calculation tool are introduced, together with some practical calibration data which indicates the effectiveness of the proposed methods.