Abstract: The exemplified systems, and method thereof, provide a transdermal or transcutaneous, non-invasive sensor system that measures blood carbon-dioxide levels and blood oxygen levels, via optical sensors, to determine an estimate of metabolic rate to provide a user of the system with a direct, continuous metabolic rate measurement. In some embodiments, the exemplified sensor system employs low-cost components such as LEDs and photodiodes to provide such direct, continuous metabolic rate measurement.