This paper presents an efficient integral-equation (IE) technique for analysis of thick irises inside multilayered rectangular waveguides. The IEs remain the same as in the case of zero-thickness iris and the thickness is accounted for only as a correction term in the IE kernel. This technique halves the number of unknowns on the iris, thus leading to computational effort and simulation times comparable to the zero-thickness case. In this paper, two efficient ways for computing the correction term are introduced and the accuracy of the approach is discussed on several waveguide structures and filters