Formula and Method for Total Body Water
Total body water (TBW) is the sum of all the water inside and outside your cells, and it typically makes up somewhere between 45% and 60% of adult body weight. This calculator uses the Watson formula (Watson, Watson & Batt, 1980), the equation most widely cited in clinical and nutrition literature for estimating TBW from easily measured variables — sex, age, height, and weight — without any lab test or bioimpedance device. It also splits the estimate into intracellular water (ICW, the fluid inside your cells) and extracellular water (ECW, the fluid in blood plasma and between cells) using the standard 2:1 physiological ratio. Because this is a fluid volume estimate, not a diagnostic test, treat the result as an educational reference rather than medical advice — always talk to a healthcare provider about fluid or hydration concerns.
How the calculation works
The calculator first converts your height to centimeters and your weight to kilograms if you entered inches or pounds. It then applies the sex-specific Watson equation: for men, TBW = 2.447 − (0.09156 × age) + (0.1074 × height in cm) + (0.3362 × weight in kg); for women, TBW = −2.097 + (0.1069 × height in cm) + (0.2466 × weight in kg). The result is in liters. Dividing TBW by your body weight in kilograms (since 1 liter of water weighs almost exactly 1 kilogram) gives TBW as a percentage of body weight. Finally, the calculator estimates intracellular water as roughly two-thirds of TBW and extracellular water as roughly one-third — the standard approximation used in physiology, since a precise ICW/ECW split actually requires bioelectrical impedance analysis.
Why results differ by sex, age, and body composition
Muscle tissue is roughly 70-75% water, while fat tissue is only about 10-20% water. Because men typically carry a higher proportion of lean muscle mass and women typically carry a higher proportion of body fat, the Watson formula gives men a higher TBW percentage (commonly 55-60% of body weight) than women (commonly 45-50%). TBW percentage also tends to decline gradually with age, largely because lean muscle mass decreases and fat mass increases over time. The Watson formula was derived from and validated on generally healthy adults roughly 18 to 100 years old; it is less reliable for children, pregnant women, or people with significant edema, dehydration, or unusual body composition (such as very high muscularity or very high body fat), where a clinician may use a different method entirely.