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Maximum stand density index (SDIMAX) models were developed for important Pacific Northwest conifers of western Oregon and Washington, USA, based on site and species influences and interactions. Inventory and monitoring data from numerous federal, state, and private forest management groups were obtained throughout the region to ensure a wide coverage of site characteristics. These observations include information on tree size, number, and species composition. The effects and influence on the self-thinning frontier of plot-specific factors such as climate, topography, soils, and geology, as well as species composition, were evaluated based on geographic location using a multistep approach to analysis involving linear quantile mixed models, random forest, and stochastic frontier functions. The self-thinning slope of forest stands dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) was found to be –1.517 and that of stands dominated by western hemlock (Tsuga heterophylla (Raf.) Sarg.) was found to be –1.461, leading to regionwide modelled SDIMAX values at the 95th percentile of 1728 and 1952 trees per hectare, respectively. The regional model of site-specific SDIMAX will support forest managers in decision-making regarding density management and species selection to more efficiently utilize site resources toward healthy, productive forests.