Total body water (TBW) is the largest single component of body composition, typically 50–65% of an adult's weight. This calculator uses the Watson formula — a widely cited 1980 regression equation — to estimate TBW in litres from age, sex, height, and weight, for anyone who wants a quick reference figure for fluid-volume or nutrition calculations.

How the Total Body Water Calculator works

The calculator applies the Watson formula, published by Watson, Watson, and Batt in the American Journal of Clinical Nutrition in 1980. The equations were derived from tritiated-water dilution studies — an isotope-based method considered a reference standard for measuring body water — in a sample of adult men and women.

There are two separate equations because men and women differ, on average, in the ratio of lean mass to fat mass at the same height and weight, and lean tissue holds substantially more water than fat tissue. The men's equation includes an age term (TBW declines slightly with age, largely reflecting age-related loss of lean muscle mass); the women's equation does not, because age was not a statistically significant independent predictor in the original female regression.

Inputs and what they mean

Age (years): used only in the men's equation. Sex: selects which of the two regression equations is applied — this is the input with the largest effect on which formula runs, though not necessarily on the numeric result for any given height/weight pair. Height: entered in centimetres or inches; taller height increases the estimate in both equations. Weight: entered in kilograms or pounds; heavier weight increases the estimate in both equations, and weight is also the denominator when converting the litre figure to a percentage of body weight.

Limits and edge cases

The Watson formula does not account for body composition beyond height and weight — it cannot distinguish a lean, muscular person from a person with higher body fat at the same height and weight, even though their true TBW percentages would differ. It was derived from an adult reference population and is not validated for children, pregnant women, or people with significant edema, ascites, or fluid-shifting medical conditions (heart failure, kidney disease, liver disease). For those situations, or whenever an accurate current hydration status is needed, a direct measurement method (bioelectrical impedance analysis, isotope dilution) or clinical assessment is required — this calculator provides only a population-average estimate, not a measurement.