Body Water Percentage Calculator
Estimate your total body water percentage based on age, gender, and weight using the Watson formula.
Body Water Content
The human body is approximately 50-65% water for males and 45-60% for females. The Watson formula (1980) estimates total body water from age, height, and weight. Body water percentage decreases with age and higher body fat. Adequate hydration is critical for temperature regulation, nutrient transport, and organ function.
About This Tool
Water makes up most of your body weight, but the exact percentage depends on age, sex, body composition, and how much fat you carry — and the formulas to estimate it aren't widely known outside clinical settings.
Enter age, sex, height, and weight, and the calculator estimates total body water using the Watson formula (the standard in dialysis and pharmacology). Output is shown in liters and as a percentage of body weight. A typical adult man is around 60% water; a typical woman is closer to 50–55% because of higher body fat percentage on average.
The Hume formula is also offered as an alternative — it's older but still cited in some clinical literature and tends to give slightly different results than Watson. Neither formula is a substitute for actual bioimpedance measurement, but for ballpark estimates and educational purposes they're consistent with the published norms.
The Watson formula was developed in the 1980s for dialysis dose calculation, where knowing total body water is essential to deliver the right dialysate volume. For men: TBW = 2.447 − 0.09156 × age + 0.1074 × height + 0.3362 × weight (with weight in kg, height in cm, and TBW in liters). For women: TBW = -2.097 + 0.1069 × height + 0.2466 × weight. The formulas are regression equations fit to measurements from real subjects, so they reflect typical body composition for the era and population studied. Modern body composition shifts (higher average body fat) introduce some drift.
The pain this addresses: figuring out hydration targets without paying for clinical bioimpedance testing. A common rule of thumb is 'drink half your body weight in ounces' — for a 180-lb person, 90 ounces, or about 2.7 liters. That's reasonable but ignores body composition entirely. A 180-lb athlete with 12% body fat has more total water than a 180-lb sedentary person with 30% body fat, and proportionally needs more daily intake. Knowing your estimated total body water gets you a more personalized starting point.
Worked example: 35-year-old man, 180cm tall, 80kg. Watson formula: 2.447 − 0.09156×35 + 0.1074×180 + 0.3362×80 = 2.447 − 3.205 + 19.332 + 26.896 = 45.47 liters. As percentage of body weight: 45.47/80 = 56.8%. That's slightly below the often-cited 60% figure for adult males, which is an average across all body compositions. Higher body fat lowers the percentage; higher lean mass raises it.
Where this can mislead: clinical scenarios. The formula's accuracy was established for relatively healthy adults. Children, pregnant women, people with significant edema (heart failure, kidney disease), people with very low body fat (under 8% for men, under 18% for women), and elderly with significant muscle wasting all deviate from the model. For these populations, bioimpedance or actual measurement is necessary. The formula is a quick estimate for normal adults, not a clinical diagnostic tool.
The about text and FAQ on this page were drafted with AI assistance and reviewed by a member of the Coherence Daddy team before publishing. See our Content Policy for editorial standards.