Abstract Aggregate height, a composite measure of seedling size and number per unit area, is proposed for describing stand density of seedling populations during early stand development. Aggregate height is analogous to other stand density measurements, such as basal area, that incorporate both size and number of trees into a single value. A large scale dataset (3452 subplots in 33 stands) from the central Appalachians and a long-term dataset (six stands from pre- to 20-years post-harvest) from the Missouri Ozarks were used to compare the utility of number of seedlings, mean height, aggregate height squared, and aggregate height for examining patterns of stand development. Comparisons of coefficients of determination ( r 2) indicate that aggregate height produces more deterministic models for describing the early development of tree seedling populations than do number of seedlings, mean height, and aggregate height squared. Stand developmental paths based on number of seedlings, basal area, and aggregate height reveal that the onset of the self-thinning stage coincides with the shifting of mean stand aggregate height, but not other measurements. The results indicate that aggregate height is an efficient measure of stand density to describe the relative prevalence of a species at a given stage of development and its ability to persist into future stages.