A method of determining respiratory rate from a photoplethysmograph signal includes filtering the photoplethysmograph signal to create a filtered photoplethysmograph signal. The method also includes detecting peaks and valleys in the filtered photoplethysmograph signal to extract predetermined features from the filtered photoplethysmograph signal. The method also includes obtaining variation waveforms from the predetermined features. The method also includes removing outliers from each of variation waveforms. The method also includes interpolating for the outliers removed from each of the variation waveforms to acquire a frequency estimation of each of the variation waveforms. The method also includes determining a respiratory rate based on the frequency estimation of each of the variation waveforms. The method also includes determining a weighted average of the respiratory rate the variation waveforms to determine an estimated respiratory rate.