Total body water calculator

How is total body water estimated?

With the Watson formulas (1980), built from measurements in adults. Men: TBW = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × weight (kg). Women: TBW = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg). The result is in liters.

Example

A 40-year-old man, 178 cm (5 ft 10 in) and 80 kg (176 lb): 2.447 − 3.806 + 19.117 + 26.896 = 44.7 liters, about 56% of body weight.

Why doesn’t age appear in the women’s formula?

The published Watson equation for women uses only height and weight; the men’s equation also includes age.

Why does body water differ between people?

Lean tissue holds much more water than fat, so the share of weight that is water is lower in people with more body fat.

How accurate is it?

It is an estimate for a typical adult of the same size, age and sex. Direct measurement (such as isotope dilution) is needed for an exact value.

How much does age change the result for men?

The male formula subtracts 0.09516 liters for every year of age. A man of 178 cm and 80 kg has 44.7 L at age 40 and 42.8 L at age 60, a difference of 1.9 L (55.8% against 53.4% of body weight). The female formula in the published equation has no age term.

What is total body water used for?

It is the volume of fluid in which drugs, salts and urea are distributed, so it appears in some clinical calculations, such as dosing and the estimate of urea volume used in dialysis adequacy. Clinicians use it with other information. The figure on this page is not a diagnosis and does not say how much water you should drink.

The Watson equations

Watson, Watson and Batt published regression equations in 1980 that predict total body water from measurements in healthy adults (Am J Clin Nutr 33:27-39). For men: TBW (liters) = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × weight (kg). For women: TBW = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg). The calculator converts feet, inches and pounds to centimeters and kilograms before applying them, and it also expresses the result in US quarts and as a percentage of body weight. The equations describe an average adult with the given measurements, not an individual.

Worked example: a man and a woman

A 40-year-old man, 178 cm and 80 kg: 2.447 − 0.09516 × 40 + 0.1074 × 178 + 0.3362 × 80 = 44.7 L, which is 55.8% of body weight. A 35-year-old woman, 165 cm and 65 kg: −2.097 + 0.1069 × 165 + 0.2466 × 65 = 31.6 L, or 48.6% of her weight. At the same height, raising her weight to 95 kg gives 39.0 L, but the share of weight falls to 41.0%, because the extra weight is assumed to be partly fat, which holds little water.

Why the percentage changes

Lean tissue is mostly water and fat is not, so a person with more body fat has a lower water share of body weight. The equations capture this: the coefficient on weight is well below the average water share, so the percent falls as weight rises at a fixed height. Women have on average a higher fraction of body fat than men of the same size, and the female equation gives a lower share. Those are averages. The equations cannot see a person's actual composition.

Common mistakes

Using the equations for children or for people with unusual body composition, such as athletes or those with edema, ascites or very high body fat, gives values that can be off by several liters. Reading the result as how much water to drink is a mistake: total body water is the water already in the body, and daily intake is a different question. Mixing up units (inches for centimeters) is another, so check that the unit toggle matches what you typed. Finally, a precise value requires direct measurement, such as isotope dilution or bioimpedance, and bioimpedance scales are themselves estimates.

Sources: P. E. Watson, I. D. Watson, R. D. Batt, "Total body water volumes for adult males and females estimated from simple anthropometric measurements", American Journal of Clinical Nutrition 33 (1980) 27-39. This page gives general information, not medical advice. Related: water intake calculator, body surface area calculator and blood volume calculator.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.