Abstract: A computer-implemented method is presented for quantifying hydration in a subject. The method includes: receiving a heart rate signal indicative of heart rate of the subject; extracting features from the heart rate signal, where one or more of the extracted features include a frequency domain representation of the heart rate signal; constructing a feature vector from the extracted features; and quantifying hydration status of the subject as a percent of body weight of the subject by classifying the feature vector using machine learning, where percentages of body weight are expressed in increments on the order of one percent or less.