A novel analysis method of Fabry-Pérot resonances of a tissue-loaded quartz window quantifies skin layer thickness and water content using terahertz spectroscopy. The analysis method treats the superficial skin layer as an unknown matching layer between the quartz window and the deep epidermis. Resolution of the longitudinal modes maps resonance frequency, 6-dB resonance width and depth to the epidermal layer water content and thickness. Validation was achieved with a telecentric beam scanning system operating in the 330 – 500 GHz band. Stinging nettle exposure to the skin promoted changes in epidermal layer thickness and moisture content for the validation measurements.