Background: Renal cell carcinoma is most commonly diagnosed as an incidental mass detected on cross sectional imaging. When a renal mass is thought to be cancerous, size on imaging is important for clinical decision-making, as there is a known positive correlation with increasing size prognostic outcomes. Image quality may be adversely impacted by a variety of factors, and we sought to assess the impact of body mass index (BMI) on the ability to obtain accurate renal mass sizes across multiple imaging modalities. Methods: A retrospective multi-institutional chart review was performed for adult patients undergoing partial or radical nephrectomy between January 2018 and December 2021 from three different hospitals and six operating surgeons. BMI groups were assigned by BMI 1: <18.5, BMI 2: 18.5-24.9, BMI 3: 25-29.9, BMI 4: 30-34.9, and BMI 5: >35. Differences between greatest dimensions were compared between pathological measurement and each imaging modality: computerized tomography (CT) with and without contrast, ultrasound (US), and magnetic resonance imaging (MRI). Paired t test was used to analyze the total cohort mean difference testing for a difference between modalities and pathology report. T-tests and Wilcoxon signed-rank tests were used to compare the different BMI groups. Results: A total of 234 patients met inclusion criteria and were identified by age, gender, procedure, and renal mass location. There was a statistically significant difference between the greatest dimension as measured on CT with contrast and MRI compared to pathological measurement in the total cohort. Sub analysis of BMI groups showed the differences in size in BMI group 4 (mean (SD)) (0.46 cm (0.52)) and BMI group 5 (0.4 cm (0.51)) were found to have a significant difference in size estimates compared to BMI 3 for CT with contrast (Table). There was no difference for size estimates by BMI group for any other imaging modality. Conclusions: While CT with contrast and MRI are important modalities for their tissue enhancement abilities, they may not be the strongest choice for strict renal mass size estimation. Specifically, CT with contrast becomes less accurate at estimating mass size for patients with BMI >30.