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In order to estimate self- and mutual inductances of the input coil and the pick-up loop of a YBCO SQUID amplifier operating at 77 K, a simple method based on the determination of the current and flux distributions into the superconductors is presented. The cases of inductors (single loop and spiral) and induced circuits (closed and open loop) are considered. The authors stress the difference between a SQUID pick-up loop and a directly coupled magnetometer depending upon their action on the input inductance. Finally, the coupling efficiency and the ground plane effect are calculated and measured for various patterns.