Total Body Water Calculator
Estimate your total body water using the validated Watson formula. Enter sex, age, height, and weight for TBW in liters, kilograms, and body water percentage.
About This Calculator
The Total Body Water Calculator estimates the volume of water in your body using the validated Watson formula, developed by Dr. P.E. Watson and colleagues in 1980. Total body water (TBW) is a key health metric used to assess hydration status, guide fluid replacement therapy, and calculate medication dosing.
This calculator uses two formulas depending on sex: for males, TBW = 2.447 − 0.09156 × age + 0.1074 × height + 0.3362 × weight; for females, TBW = −2.097 + 0.1069 × height + 0.2466 × weight. While a simple estimate can be made using the average 60% for men and 50% for women, the Watson formula accounts for individual differences in age, height, and weight to provide a more accurate result.
Body water is distributed across compartments: intracellular fluid (ICF) makes up about two-thirds of total body water, while extracellular fluid (ECF) makes up the remaining one-third. ECF is further divided into interstitial fluid (fluid between cells) and plasma (the liquid component of blood).
Regional Notes
India: Height and weight are commonly measured in cm and kg in clinical settings across India. The normal TBW range applies to all populations regardless of region.
United States: If your height is in feet/inches, convert to cm (1 inch = 2.54 cm). If your weight is in pounds, convert to kg (1 lb = 0.4536 kg). The calculator accepts metric inputs.
United Kingdom: The NHS uses metric measurements (cm and kg) for body composition assessments. The Watson formula is referenced in clinical nutrition guidelines across the UK.
Disclaimer: This calculator provides reference information only and is not a substitute for professional medical advice, diagnosis, or treatment.
Frequently Asked Questions
What is the Watson formula for total body water?
The Watson formula estimates total body water using sex, age, height, and weight. For males: TBW (L) = 2.447 − 0.09156 × age + 0.1074 × height (cm) + 0.3362 × weight (kg). For females: TBW (L) = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg).
What is a normal total body water percentage?
The normal total body water range is 58 ± 8% for males (50–66%) and 48 ± 6% for females (42–54%). These ranges vary with age, body composition, and muscle mass.
How is total body water distributed in the body?
Total body water is divided into intracellular fluid (ICF, about 2/3 of TBW) found inside cells, and extracellular fluid (ECF, about 1/3 of TBW). ECF is further divided into interstitial fluid (4/5 of ECF) surrounding cells and plasma (1/5 of ECF) in blood vessels.
How accurate is the body water calculator?
This calculator uses the Watson formula, which is validated for estimating total body water in adults of all ages. Results have tight confidence intervals but should be used as estimates. Always consult a healthcare professional for clinical decisions.
Can I use this calculator for children?
The Watson formula is validated for adults only. For pediatric patients, the Hummel formula or other age-specific equations are more appropriate. Consult a pediatrician for accurate assessment in children.
How does muscle mass affect body water percentage?
Muscle tissue contains approximately 70% water, while fat tissue contains only about 10% water. Individuals with higher muscle mass typically have a higher total body water percentage. This is one reason why men generally have a higher water percentage than women.
Can total body water change throughout the day?
Yes, total body water fluctuates by about 0.5% throughout the day due to fluid intake, sweating, urination, and respiration. Hot weather, exercise, and illness can cause larger fluctuations. The body maintains fluid balance through thirst and hormonal mechanisms.
How do I increase my body water percentage?
To increase body water percentage, drink adequate water throughout the day, eat fresh fruits and vegetables with high water content, and build muscle mass through strength training since muscle holds more water than fat. Using a hydration tracking app can help maintain consistent fluid intake.