Body Water Calculator

Background.

Water is the largest single component of the body, roughly 60% of a lean adult man's weight and nearer 50 to 55% of a woman's, and it is not spread evenly. Lean tissue such as muscle is about 73% water while fat holds very little, so two people of the same weight can carry quite different volumes depending on how much of that weight is fat.

Measuring it properly means drinking a dose of isotope-labelled water and tracking how far it is diluted, a research technique. In 1980 Watson and colleagues used measurements of that kind to fit equations that predict total body water from things anyone can record: sex, age, height and weight. Those equations are still the usual bedside estimate. Dialysis teams use them for urea's distribution volume, the V in Kt/V, and they turn up in some blood-alcohol estimates.

The two equations behave differently. For men, predicted water rises with height and weight and falls with age, consistent with the gradual loss of muscle across adult life. The women's equation has no age term at all; only height and weight enter. Both give more litres to a heavier person, but the percentage of body weight usually falls as weight rises, because each extra kilogram is credited with less water than the body's average.

The result is a population estimate for adults, not a measurement of you. It cannot see oedema, dehydration, pregnancy or an unusually muscular build, and it is no basis for prescribing fluids. Anyone whose hydration is clinically important needs assessment by the people treating them.

What is body water calculator?

Total body water is the volume of all the water in the body, inside cells and outside them, expressed in litres. About two-thirds sits inside cells; the remaining third is extracellular, in plasma and in the fluid that bathes tissues. This page estimates the total with the Watson regression equations for adults and also expresses it as a percentage of body weight, treating one litre of water as weighing one kilogram.

How to use this calculator.

1. 01Select the Male or Female equation. Watson fitted them separately, and the female one does not use age.
2. 02Enter age in years (adults 18 and over only), height in centimetres and weight in kilograms. For reference, 5 ft 10 in is 177.8 cm and 176 lb is about 79.8 kg.
3. 03Use your usual weight in light clothing. A weight taken during visible swelling, or after rapid weight loss from illness, will mislead the estimate.
4. 04Read the total in litres and the percentage of body weight, and treat differences of a litre or so as within the error of any prediction equation.

The formula.

Men: TBW = 2.447 − 0.09516 × age + 0.1074 × height + 0.3362 × weight. Women: TBW = −2.097 + 0.1069 × height + 0.2466 × weight. TBW is in litres, age in years, height in centimetres and weight in kilograms. The percentage is TBW ÷ weight × 100, taking a litre of water to weigh a kilogram.

The coefficients are regression slopes, fitted by Watson and colleagues to adults whose body water had been measured by isotope dilution. In the male equation each extra kilogram adds about 0.34 L, each extra centimetre about 0.11 L, and each year of age removes about 0.095 L, so a decade subtracts close to a litre. In the female equation a kilogram adds about 0.25 L, which is why the same height and weight produce less water for a woman: on average a larger share of women's weight is fat. Weight is the input that moves the answer most.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Watson et al.. American Journal of Clinical Nutrition 1980. Adult total body water regression equations (PAYWALL). Retrieved 2026-08-06. access: gated.
2. [2]U.S. National Library of Medicine. PubMed record current 2026-08-06. Indexed derivation and validation literature. Retrieved 2026-08-06.
3. [3]World Health Organization. Patient safety resources current 2026. Clinical decision-support safety context. Retrieved 2026-08-06.