The increasing use of microstrip antenna technology requires methods of accuratelly predicting the input impedance. Our purpose is to derive a procedurefthat can easy and accuratelly define the above impedance. It is used the view point of plane waves that propagate under the microstrip patch, bounding back and forth from the edges and interacting in a constructive manner. The reflection coefficient Γ(α)=ejx(α) at the edges needs long computation, time for exact values while the approximate values give significant error in the impedance. We correct the approximate expression of Γ(α) and give a polynomial one which has the same values with the exact. Sveral numerical and experimental results show the validity of our expressions.