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Tree-level models used in large-scale inventories necessitate flexible modeling structures that can accommodate multiple species across varying sites and regions. Presently the United States Forest Service, Forest Inventory and Analysis Program (USFS-FIA) utilizes over 40 volume models of different forms specific to 22 geographic regions. Recently much of the data used to create these volume models has been compiled into a database including nearly 250,000 trees. This database provides the opportunity to formally assess tree-level variation in volume and taper among and within eco-physical regions and at varying taxonomic levels. In this analysis, we developed national-scale, non-linear, mixed effects outside bark taper and bark-thickness models to calculate merchantable volume in a consistent manner between regions and taxonomic groups. We assessed these models in terms of accuracy and precision and compared them to merchantable volume models currently used by FIA. The mixed effects taper-based approach performed well when compared to the conventional approach for estimating merchantable wood volume. Using a single, wide scale volume modeling system should lead to improved estimates of volume for some species particularly where little data is available.