Total Body Water Calculator
Total body water (TBW) is the total volume of water in the body, roughly half to two-thirds of body weight. It is used in pharmacokinetics, dialysis dosing, and fluid management. This calculator uses the Watson formula, a widely cited anthropometric estimate based on age, height, weight, and sex. It is an estimate from population equations and is not a measurement; clinical use requires professional judgement.
Watson total body water formula
Male TBW (L) = 2.447 - (0.09156 * age) + (0.1074 * height_cm) + (0.3362 * weight_kg)
Female TBW (L) = -2.097 + (0.1069 * height_cm) + (0.2466 * weight_kg)
Age is in years, height in centimetres, and weight in kilograms. The female equation does not include an age term. Percent of body weight is TBW divided by weight, since one litre of water weighs one kilogram.
Worked example
For a 40-year-old male, 175 cm, 80 kg: TBW = 2.447 - (0.09156 * 40) + (0.1074 * 175) + (0.3362 * 80) = 2.447 - 3.662 + 18.795 + 26.896 = 44.48 L, which is 44.48 / 80 = 55.59% of body weight.
Frequently asked questions
What is total body water used for?
TBW is used in clinical pharmacology to estimate the volume of distribution of water-soluble drugs, in nephrology to guide dialysis adequacy, and in fluid and electrolyte management. The Watson estimate is one of several anthropometric formulas.
Why does sex change the formula?
On average, women carry a higher proportion of body fat, which contains little water, so total body water as a fraction of weight is lower. The Watson equations use separate coefficients for each sex to reflect this.
Is this the same as body water percentage?
It is related. This tool returns an absolute volume in litres and also the percentage of body weight. A dedicated body water percentage tool may use different reference equations.
Is the result a measurement?
No. It is a statistical estimate from a population equation. Actual TBW varies with hydration, body composition, and disease. Use measured methods for clinical precision.
Sources
- Watson PE, et al., total body water equations, indexed at the U.S. National Library of Medicine: PubMed record.
- U.S. National Library of Medicine, MedlinePlus: Fluid and Electrolyte Balance.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. Educational estimate, not medical advice. See our methodology.