This paper proposes a new standalone iterative frequency setting algorithm based on adaptive step size to achieve convergent frequencies for a given route set. The proposed algorithm aims to assign adequate frequencies to the routes to minimize user cost (travel time). Particularly, frequencies are determined based on passenger flow, which is obtained by assigning passengers based on an assumed behavior when choosing routes. The behavior of passengers in choosing routes depends on the frequencies of routes. Thus, this process is repeated till the headway difference is converged for each route. During these iterations, we update the frequency conservatively by using (i) last seen frequency, (ii) difference between the last and desired frequencies, and (iii) an increment parameter. The value of this parameter is decided depending on two cases as follows. Firstly, the loop never ends if similar frequencies oscillate during non-consecutive iterations. Such cycling is overcome by using a predefined step size as an increment parameter. Moreover, the step size reduces with each occurrence of cycles. Secondly, if no cycle exists but convergence is not achieved until a relatively higher number of iterations, then the frequencies are updated by a rounding factor. The proposed algorithm is tested on Mandl and Sioux Falls networks. It is observed that frequencies for various route sets are converging within a reasonable number of iterations. Our algorithm is useful for strategic and tactical planning, especially when (i) frequencies are to be determined for an already existing route set and (ii) if routes are added, removed, or modified, then computing new frequencies for the altered route set.