Total Body Water Calculator
Enter your sex, age, height and weight to estimate your total body water (TBW) using the validated Watson or Hume-Weyers formula. Results appear in litres, as a percentage of body weight, and broken down into intracellular and extracellular fluid compartments. Switch between metric and imperial units; everything updates as you type.
What is total body water?
Total body water (TBW) is the total volume of water in the human body across all tissues, organs and fluids. In a healthy adult, water makes up about 50-60% of body weight - more in lean individuals because muscle tissue contains roughly 75% water, while fat tissue holds only about 10%. TBW is divided into two main compartments: intracellular fluid (ICF), the water inside cells, which accounts for about two-thirds of TBW, and extracellular fluid (ECF), the water outside cells, which makes up the remaining one-third. ECF is itself split into plasma (the liquid part of blood, about 1/5 of ECF) and interstitial fluid (the fluid bathing cells, about 4/5 of ECF). Keeping TBW within a normal range is essential for cell function, nutrient transport, thermoregulation and blood pressure control.
Watson formula vs. Hume-Weyers formula
This calculator offers two validated formulas for adults. The Watson (1980) formula uses sex, age, height and weight; it is the most widely used in clinical pharmacokinetics and drug dosing because it was validated in large adult cohorts and produces reliable estimates for volume-of-distribution calculations. The Hume-Weyers (1971) formula uses sex, height and weight but not age; it was derived from tritium dilution studies and is common in European clinical literature. The two formulas typically agree within 1-2 litres for most adults. Neither formula is validated in children under 18, pregnant women, extreme obesity, or patients with oedema or ascites, where direct measurement by bioelectrical impedance analysis (BIA) or isotope dilution is preferred.
Why body water percentage varies between people
Body water percentage is not a fixed number; it is strongly influenced by body composition, sex, age and health status. Because fat tissue holds very little water, individuals with higher body fat tend to have a lower TBW percentage. Adult men average about 60% and adult women about 50%, largely because women typically carry a higher proportion of adipose tissue. Body water percentage also declines gradually with age as lean muscle mass decreases. Athletes and people with high muscle mass tend to have percentages at the upper end of the range. Clinical dehydration, oedema, ascites and certain chronic diseases all shift TBW away from the estimated values these formulas produce.
How TBW is used clinically
Clinicians use TBW estimates to calculate drug doses for medications that distribute primarily in body water (such as aminoglycosides and lithium), to guide fluid replacement after dehydration or haemorrhage, and to interpret serum sodium abnormalities. In nephrology, TBW underpins the Kt/V calculation used to assess dialysis adequacy. The free-water deficit in hypernatraemia is calculated directly from TBW. Because these applications can directly affect patient care, any estimate from an anthropometric formula should be interpreted alongside clinical signs and laboratory values.
Body water percentage reference ranges by age and sex
| Age group | Males (approx.) | Females (approx.) |
|---|---|---|
| Newborn | 75-80% | 75-80% |
| 1 year | ~65% | ~65% |
| Children 3-10 | 62% | 60% |
| Adolescents 11-17 | 60% | 55% |
| Adults 18-60 | 58-60% | 50-55% |
| Older adults 61+ | 54-57% | 46-50% |
Normal-weight reference values. Body water percentage falls with increasing body fat; higher muscle mass raises it.
Frequently asked questions
How is total body water calculated?
The most widely used method is the Watson formula, which uses sex, age, height and weight in a regression equation derived from isotope dilution studies. For men: TBW = 2.447 - 0.09156 x age + 0.1074 x height (cm) + 0.3362 x weight (kg). For women: TBW = -2.097 + 0.1069 x height (cm) + 0.2466 x weight (kg). The Hume-Weyers formula is an alternative that omits age. Both produce results in litres.
What is a normal total body water percentage?
For healthy adults, total body water typically represents about 55-60% of body weight in men and 45-55% in women. The difference is mainly because women carry a higher proportion of fat tissue, which contains very little water. Values outside this range are common in athletes (higher, due to more muscle) and in people with obesity (lower, due to more fat). Body water percentage also declines gradually from birth (around 75-80%) through old age.
Why is body water percentage lower in females than males?
Adult women typically have a higher percentage of body fat than men of the same age and body weight, and fat tissue contains only about 10% water compared with 75% in muscle. This compositional difference accounts for the roughly 10 percentage-point gap in average TBW percentage between sexes. Hormonal factors, including oestrogen effects on fat distribution, also play a role.
Does total body water decrease with age?
Yes. TBW percentage decreases gradually from infancy through old age. Newborns are approximately 75-80% water. By adulthood the percentage falls to around 60% in men and 50% in women, and it continues to decline in older adults - partly because lean muscle mass decreases with age, and partly because older adults often have a higher proportion of fat tissue.
Can I use this calculator for children?
The Watson and Hume-Weyers formulas were validated in adult populations and are not accurate for children. For paediatric patients, dedicated equations such as the Mellits-Cheek formula are used; these have separate coefficients based on sex and height thresholds. Clinical TBW assessment in children is usually done with bioelectrical impedance analysis rather than anthropometric equations.
How does dehydration affect total body water?
Dehydration reduces TBW. Mild dehydration (a 1-2% loss of body water) causes thirst and reduced urine output; moderate dehydration (3-5% loss) impairs physical and cognitive performance; severe dehydration (above 8-10% loss) is a medical emergency. Because the formulas in this calculator use anthropometric measurements taken at one point in time, they estimate TBW under usual conditions and cannot directly detect acute changes in hydration status.