Total Body Water Calculator — Body Water Percentage & Liters EstimatorWatson Formula · TBW · ICF · ECF · Hydration Status · Men & Women
Use this free Total Body Water (TBW) Calculator to accurately estimate the total volume of water in your body — measured in both liters and as a percentage of total body weight — based on your age, gender, height, and weight using the clinically validated Watson Formula for Total Body Water, the most widely referenced TBW estimation equation in medical and sports science research. Total body water typically constitutes 45–60% of body weight in adult women and 50–65% of body weight in adult men, distributed across two primary fluid compartments: Intracellular Fluid (ICF) — the water held within cells, accounting for approximately 60% of total body water — and Extracellular Fluid (ECF) — comprising blood plasma, interstitial fluid, and lymphatic fluid. Maintaining optimal body water percentage is essential for cellular hydration, cardiovascular circulation, electrolyte balance, thermoregulation (body temperature control), kidney function and toxin elimination, joint lubrication, and peak cognitive and athletic performance. Deviations from your healthy TBW range may indicate dehydration, overhydration (hyponatremia), edema, or underlying metabolic and renal health conditions. This body water percentage calculator is used by sports scientists, clinical dietitians, nephrologists, and fitness professionals to assess hydration status, monitor fluid retention, and optimize daily water intake recommendations for better health, recovery, and physical performance.
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Body Water Calculator — Total Body Water From the Watson and Hume-Weyer Formulas
Total Body Water (TBW) is the amount of water in the body as a fraction of body weight, typically 60% in adult males and 50-55% in adult females (women have more body fat, which holds less water than muscle). The Watson formula calculates TBW in liters: for men, TBW = -2.097 + 0.1069×height(cm) + 0.2466×weight(kg); for women, TBW = -2.097 + 0.1069×height(cm) + 0.2466×weight(kg) with female-adjusted coefficients. A 180 cm, 80 kg male has approximately 43.5 liters of TBW. The body water calculator implements both Watson and Hume-Weyer formulas with a comparison of their outputs.
Body water distributes between intracellular fluid (65%) and extracellular fluid (35%), with extracellular fluid further divided between plasma and interstitial fluid. Dehydration first depletes extracellular fluid, which triggers thirst at a loss of roughly 1-2% of body weight. At 2% dehydration, cognitive performance measurably declines. At 5%, heat cramps and nausea appear. Athletes exercising in heat can lose 1-1.5 liters/hour in sweat — equivalent to 1.5-2% of body weight per hour for a 70 kg person. The calculator contextualizes TBW against hydration loss rates for exercise planning.
TBW measurement in clinical settings (via deuterium dilution or bioimpedance) is used to calculate drug dosing for water-soluble medications. Aminoglycosides, some antibiotics, and digoxin are dosed based on actual or ideal body weight with TBW as the distribution volume. Obese patients have more absolute TBW but lower TBW as a percentage of body weight — which affects volume-of-distribution calculations for hydrophilic drugs. The body water calculator is relevant for pharmacokinetics applications beyond general health assessment.