Allometric equations are widely used to estimate forest aboveground biomass (AGB). However, their development rarely includes the oldest and largest trees, leading to estimation errors. Black locust (Robinia pseudoacacia) is an early successional nitrogen-fixing tree, native to the Eastern United States. It is widespread, often the dominant tree following disturbance, and can be a significant source of new nitrogen to recovering forests. Here we developed allometric equations for black locust to predict AGB and leaf area based on diameter at breast height (DBH). We compiled existing data from our study site and sampled new trees, ranging in size from 6.0-58.5 cm DBH. Destructively harvested new trees were measured for foliage, branch, and bole dry biomass and carbon and nitrogen concentrations. Parameters for our predictive equations were lower than those previously published; existing equations applied to these largest individuals resulted in overestimates of bole and branch biomass on average by 33.6 and 325.3 percent, respectively. We also found that foliage and woody nitrogen concentrations declined with age, together suggesting age-related declines in black locust are greater than other co-occurring species. Our equations significantly improved accuracy of AGB predictions and will aid in site-specific forest biomass estimates and new nitrogen inputs.