Significant weight loss among teens may indicate eating disorder (ED) onset, and weight restoration and/or stabilization is crucial for successful treatment and sustained recovery from restrictive EDs. While achieving and sustaining a restored body weight is necessary during ED recovery, it is often challenging to define an individual's expected healthy body weight (EBW) due to diverse premorbid growth trajectories. Population-based norms fall short in capturing the nuances of illness and recovery across individuals, and there are no current tools that offer personalized, reproducible, and statistically-driven EBW estimations. This paper introduces the TeenGrowth package and its web-based application, designed to calculate and forecast expected body mass index (eBMI) and EBW across adolescence. With input from individual growth charts, demographic data, and age of ED onset, TeenGrowth offers a reproducible method for determining EBWs. The package includes functions for data cleaning, BMI and BMI z-score calculations, and growth forecasting, accommodating various data inputs. The accompanying Shiny web application provides a user-friendly interface for researchers and providers, enabling the identification of EBWs for individual patients and weight restoration planning. Through a series of simulation studies, the package's various options for predictive models are evaluated, highlighting its potential for use in ED screening and improving accuracy of EBW estimations. The introduction of TeenGrowth represents a significant step towards personalized medicine in ED treatment, with potential to promote individualized care and enhance clinical outcomes.