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A simple mathematical formula is proposed for computing the coupling coefficient between two arbitrary antennas that are placed within the other's near-field region. The information required by this expression consists of the associated normalized vector far-field patterns, their relative orientations, and the antenna spacing. To validate our proposed expression, coupling coefficients in several near-field scenarios are computed, including for the case of a practical near-field ultra-high frequency (UHF) radio-frequency identification (RFID) system. These results are then compared to practical measurements and to the outcome of full-wave simulations generated using Ansoft HFSS. All results are in good agreement. Additionally, in this paper, it is shown that several factors may influence the coupling coefficient, such as the impedance matching of the receiving antenna and the directivities of both antennas. With the aid of our formula, the near-field read range can be determined and near-field coupling phenomena can be investigated. The results thus obtained may be useful in near-field communication systems.