The calibration factors of synthetic aperture radar (SAR) images are generally obtained by estimation of radar system parameters based on internal calibration and external calibration. In this paper, we propose a simple but efficient method to estimate the calibration factors based on statistical modeling of scattering coefficient. Taking expectation and variance on the linear form of calibration equation, we derive the analytical expressions of such estimator. Modeling the scattering coefficient as Rayleigh, heavy-tailed Rayleigh, log-normal, and Weibull distributions, respectively, we obtain the calibration factor estimators for two kinds of radar receivers: the linear receiver and the square-law receiver. Lastly, Monte Carlo simulation results are provided to demonstrate the efficiency of the proposed calibration factor estimator.