The transmit antenna factor of a dipole antenna in a tissue-equivalent liquid phantom is derived from the power transmission formula and used in the calibration of an electric field probe for specific-absorption-rate (SAR) measurements of wireless power transfer systems operating in the megahertz band. First, we determine the length of the reference dipole antenna to be used in the calibration by the method of moments. Then, we construct a calibration system, including in-house fabricated dipole antennas and a phantom liquid with a concentration of 0.074 mol/L. The transmit antenna factor is then determined from the measured S-parameters and the conversion factor (CF) of the probe immersed inside the liquid phantom is derived by using our proposed calibration procedure. The CF is compared with that provided by a manufacturer to confirm the validity of our calibration method. An uncertainty budget is provided and the difference between the manufacturer's value and our result is indicated to be within 2.6%, less than the expanded uncertainty, indicating the validity.