Computation procedure of an interdigital transducer (IDT) input admittance, Yin, from a 2D-Finite-Difference Time-Domain (FDTD) method, is presented. The IDT is intended to be used as the transducer of a surface acoustic wave (SAW) RFID tag. A piezoelectric substrate of 128°YX-LiNbO3 is used. Since input admittance is a frequency-domain concept, it can be computed from acoustic and electric fields obtained from the FDTD simulation, only after they are transformed into the frequency domain. A Fast Fourier Transform (FFT) algorithm is used for this purpose. With the frequency-domain fields, the total power flux generated by the IDT and the voltage at its terminals can be computed. Finally, the input admittance is determined from these two quantities. The computed admittance is compared with theoretical values.