Total Body Water (TBW)
Estimate total body water volume using the Watson formula (Am J Clin Nutr, 1980), used in fluid balance and dialysis planning.
Results
What is it?
Total Body Water (TBW) estimates the volume of water in your body using the Watson formula (Am J Clin Nutr, 1980). Water typically accounts for 50–65% of body weight and is critical for fluid balance, renal dosing, dialysis planning, and electrolyte management.
How to use
Enter your weight (kg or lb), height (cm or in), age, and biological sex. Males: TBW = 2.447 − 0.09156 × age + 0.1074 × height + 0.3362 × weight. Females: TBW = −2.097 + 0.1069 × height + 0.2466 × weight.
Example scenario
A 30-year-old male, 75 kg, 175 cm: TBW = 2.447 − 0.09156 × 30 + 0.1074 × 175 + 0.3362 × 75 = 2.447 − 2.747 + 18.795 + 25.215 = 43.7 L (≈ 58.3% of body weight).
Pro tip
TBW is essential for calculating free-water deficit in hypernatraemia: deficit = TBW × ([Na⁺]/140 − 1). In dialysis, TBW helps set ultrafiltration targets. Remember that TBW decreases with age and higher body fat percentages.