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Recently, unequal-spaced channel-allocation techniques have been studied and analyzed to reduce four-wave-mixing (FWM) crosstalk in high-capacity, long-haul wavelength-division multiplexing (WDM) transmission systems. It will be helpful to system designers if there exists a fast tool to measure the strength of FWM crosstalk in such WDM systems. In this paper, we develop two algorithms, using frequency difference triangles and frequency difference squares, that can determine the total numbers of FWM signals falling onto the operating band and each channel, respectively, of unequal-spaced WDM systems. By knowing these two numbers, one can adjust the system parameters, such as minimum channel spacing, in order to keep a good balance on the adverse effects of FWM crosstalk and interchannel interference, or avoid the assignment of channels at locations with the most severe crosstalk.