1. Background and Rationale: With the growing number of overweight and obese Americans, health professionals need accurate and convenient tools to help personalize weight loss programs. Portable indirect calorimetry measurements are more expensive than handheld devices and often limited to research setting only, making them impractical for clinical use. Additionally equations used to estimate resting metabolic rate may be significantly inaccurate in overweight patients. Several studies have been conducted to determine the accuracy of handheld devices, however none have been done in the overweight and obese populations. If handheld calorimeters can accurately measure resting metabolic rate in overweight and obese patients, these tests may be used to improve and personalize weight management programs.