Objectives: Adults with decreased muscle mass experience worse outcomes and more frequent complications. The effects of sarcopenia on pediatric outcomes are unknown. Our objective was to define reference values for lean muscle mass in a healthy pediatric population to facilitate future studies on the impact of lean muscle mass on pediatric outcomes. Patients and Methods: Bilateral psoas muscle surface area was measured by computed tomography in a healthy pediatric population undergoing evaluation after trauma. Pearson correlation coefficients (PCCs) were calculated for age, weight, height, body mass index (BMI), total psoas muscle area, and psoas muscle index (PMI; defined as psoas muscle area divided by height squared). Quantile regression was used to determine age- and sex-specific percentiles of psoas muscle area and PMI. Results: Analysis of 494 male and 288 female patients with available imaging (median age: 9.3 years, interquartile range: 5.4–13.4; 63.1% male) was performed. For males, age correlated strongly with total psoas volume (PCC = 0.87), height (0.95), and weight (0.88) and poorly with BMI (0.45). In females, age correlated strongly with total psoas volume (0.88), height (0.92), weight (0.88) and poorly with BMI (0.19). Gender-specific curves and charts were created using output from the quantile regression from reference values of the total psoas muscle area corresponding to the 25th, 50th, and 75th percentiles across all ages. Conclusions: We created gender-specific reference charts for total and height-normalized psoas muscle area in healthy children based on age. These results can be used in future studies to establish the effects of sarcopenia in pediatric patients.