Undiagnosed dehydration compromises health outcomes across many populations including the elderly, infants, soldiers and athletes. Here we introduce a miniature (~1000 cm3), portable (~4 kg), single-sided magnetic resonance (MR) sensor capable of identifying and quantifying fluid loss induced by dehydration. Multicomponent T2 relaxometry of skeletal muscle tissue enables the measurement of distinct fluid compartments. We first quantify fluid loss induced by dehydration with multicomponent T2 relaxometry using a standard MRI. We then extend these findings and demonstrate a portable, low-field MR sensor exhibits comparable performance via a similar measurement while offering significantly reduced sensor mass, measurement time, and system complexity.