An accurate measurement of BSA involved in patients injured by burns is critical in determining initial fluid requirements, nutritional needs, and criteria for tertiary center admissions. The rule of nines and the Lund-Browder chart are commonly used to calculate the BSA involved. However, their accuracy in all patient populations, namely obese patients, remains to be proven. Detailed BSA measurements were obtained from 163 adult patients according to linear formulas defined previously for individual body segments. Patients were then grouped based on body mass index (BMI). The contribution of individual body segments to the TBSA was determined based on BMI, and the validity of existing measurement tools was examined. Significant errors were found when comparing all groups with the rule of nines, which overestimated the contribution of the head and arms to the TBSA while underestimating the trunk and legs for all BMI groups. A new rule is proposed to minimize error, assigning 5% of the TBSA to the head and 15% of the TBSA to the arms across all BMI groups, while alternating the contribution of the trunk/legs as follows: normal-weight 35/45%, obese 40/40%, and morbidly obese 45/35%. Current modalities used to determine BSA burned are subject to significant errors, which are magnified as BMI increases. This new method provides increased accuracy in estimating the BSA involved in patients with burn injury regardless of BMI.