Methods and systems for calculating body fluid metrics are provided. In accordance with an exemplary embodiment of the present technique, there is provided a method for calculating body fluid metrics by acquiring an absorbance spectrum of a subject’s tissue over a range of near-infrared light, performing a multi-linear regression of the absorbance spectrum of the subject’s tissue in relation to absorbance spectra of tissue constituents, and calculating body fluid metrics based on the results of the multi-linear regression. A system is provided having a sensor for emitting the light into the subject’s tissue and detecting reflected, scattered, or transmitted light, a spectrometer for processing the detected light and generating the absorbance spectrum of the subject’s tissue, memory for storing absorbance spectra of the tissue constituents and a multi-linear regression model, and a processor for performing the multi-linear regression and calculating the body fluid metrics.