Abstract: To calculate the interelectrode capacitance, two approximate formulas of an IDT on a piezoelectric layered structure are obtained by assuming a simple potential function and a simple electric charge density function on the intersurface of IDT. For the different values of the electrode width factor a, i. e. When a<3/8, S/8<a< 6/8 and a>5/8, the approximate values of the calculated capacitance are very good.