To quantify and evaluate the impact of macroparasitism to host population, we developed an alternative mathematical model of macroparasite distribution and proposed a novel index, which we call the congestion rate. We suggest that a biologically oriented model to replace the coefficient of non-linear parasite impact in the earlier host-parasite models. The congestion rate model can estimate parasite-induced host mortality, and replace the negative binomial analysis that has been widely supported to estimate macroparasite impact. We re-analyzed the same data with our model to examine its validity. The trends and interpretations were similar for both inter/intra host populations. The advantages of the congestion rate model are its simple computation and interactive biological meaning. Our model can strengthen our understanding of macroparasitism in terms of population dynamics, host behavior, and co-evolution.