Body Water Calculator

About this calculator

This calculator estimates total body water (TBW) — the amount of fluid inside cells, blood and tissue — using the Watson formula, which factors in weight, height, age and sex (men typically carry a higher percentage of body water than women, around 60% versus 50%, due to differing muscle-to-fat ratios). Muscle tissue holds far more water than fat tissue, which is why the formula needs body composition inputs rather than weight alone.

Athletes and coaches monitoring hydration status around training, dietitians assessing a client's fluid balance, and anyone tracking body composition changes alongside weight loss use this figure as a baseline, since body water percentage shifts noticeably as fat and muscle mass change even when total weight stays the same.