Body Water Percentage Calculator
The human body is primarily composed of water, which is vital for transporting nutrients, regulating temperature, removing waste, and supporting every cellular function. Total body water (TBW) as a percentage of body weight varies by sex, age, and body composition. Lean muscle tissue holds significantly more water than adipose (fat) tissue, which is why athletes and muscular individuals tend to have higher body water percentages. This calculator uses the clinically validated Watson (1980) formula, which estimates TBW from height, weight, age, and sex. The result is shown in both litres and as a percentage of body weight, with context on healthy ranges for adults.
Watson (1980) total body water formula
Male: TBW (L) = -2.097 + 0.1069 x Height(cm) + 0.2466 x Weight(kg)
Female: TBW (L) = -2.097 + 0.1069 x Height(cm) + 0.2466 x Weight(kg) - 2.097
Note: The exact Watson coefficients for males are: TBW = 2.447 - (0.09516 x Age) + (0.1074 x Height) + (0.3362 x Weight). For females: TBW = -2.097 + (0.1069 x Height) + (0.2466 x Weight). Body water % = TBW / Weight x 100.
Body water percentage: frequently asked questions
What is total body water (TBW)?
Total body water is the total amount of fluid in the body, expressed as a percentage of body weight. It includes intracellular fluid (inside cells) and extracellular fluid (blood plasma, interstitial fluid). Water is essential for virtually all physiological processes.
What is a healthy body water percentage?
Healthy ranges vary by sex and age. Adult men generally fall between 50-65% and women between 45-60%, because women typically have a higher percentage of body fat, which contains less water than lean tissue. Athletes tend to have higher percentages.
How does the Watson formula work?
The Watson (1980) formula estimates TBW from height, weight, age, and sex using regression coefficients derived from deuterium dilution studies. For men: TBW = -2.097 + 0.1069 x Height(cm) + 0.2466 x Weight(kg). For women: TBW = -2.097 + 0.1069 x Height + 0.2466 x Weight (with different constants).
What causes low body water percentage?
Dehydration from inadequate fluid intake, excessive sweating, illness (vomiting/diarrhoea), high body fat percentage (adipose tissue is about 10% water vs 75% in muscle), ageing, and certain medical conditions can all reduce body water percentage.
Why does body water change with age?
Total body water as a proportion of body weight decreases with age due to a reduction in lean muscle mass (which holds more water) and an increase in fat tissue (which holds less water). Newborns are about 75% water; elderly adults may be as low as 45-50%.
Official sources
- Watson PE et al. (1980). Total body water volumes for adult males and females estimated from simple anthropometric measurements. American Journal of Clinical Nutrition: PubMed: 7361681.
- National Academies of Sciences: Dietary Reference Intakes for Water.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.