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Total Body Water Percentage Calculator

Enter your age, height, weight and sex to see how much of your body is water.

How it is calculated

The tool uses the Watson formula for adults, with age in years, height in centimetres and weight in kilograms. Men: TBW = 2.447 − 0.09156 × age + 0.1074 × height + 0.3362 × weight. Women: TBW = −2.097 + 0.1069 × height + 0.2466 × weight. TBW is total body water in litres, and water % = TBW ÷ weight × 100. Age matters only in the male equation, and any sex setting other than female is treated as male. Inputs are limited to ages 18–120, height 50–272 cm and weight 2–650 kg, so a child's age or a height typed in metres gives no result; so does a formula result that is zero, negative or more litres than the body weighs. Watson is a population average from adult data, so it is an estimate, not a hydration measurement.

Worked example

A 35-year-old man, 178 cm, 82 kg. First 2.447 − 0.09156 × 35 = 2.447 − 3.2046 = −0.7576. Add 0.1074 × 178 = 19.1172 and 0.3362 × 82 = 27.5684, which gives TBW = 45.93 L. Water % = 45.93 ÷ 82 × 100 = 56.01%. For a 35-year-old woman of 165 cm and 62 kg the female equation gives 30.83 L, which is 49.73%.

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Common mistakes

  • Entering height in metres or in feet and inches. The tool needs centimetres and rejects a value such as 1.78 outright, since heights below 50 cm are not accepted.
  • Entering weight in pounds. The formula needs kilograms; 180 instead of 82 gives 78.88 L and a percentage of 43.82%, both wrong.
  • Expecting the percentage to track hydration day to day. It is derived from height, weight, sex and (for men) age, so drinking water does not change it.
  • Comparing it with a bioimpedance scale reading and calling one of them wrong. They use different methods, and a Watson result is a population average for your measurements.
  • Forgetting to set the sex. The default is male, so a woman who leaves it unchanged gets the male equation and a different litre figure than she should.

Sources

About this calculator

Total body water (TBW) — the litres of fluid inside and outside your cells — is most commonly estimated with the Watson formula, which uses height, weight and sex — with age entering only the male equation — because body composition (and therefore water content) differs systematically between men and women: for men, TBW = 2.447 − (0.09156 × age) + (0.1074 × height in cm) + (0.3362 × weight in kg); for women, TBW = −2.097 + (0.1069 × height in cm) + (0.2466 × weight in kg). Dividing TBW by body weight gives the percentage figure, which typically runs around 50–65% for adults, higher in lean, muscular bodies since muscle holds far more water than fat.

Clinicians use TBW estimates to calculate a free water deficit in dehydrated or hypernatremic patients, to dose hydrophilic medications that distribute through body water rather than fat, and to guide fluid management in dialysis and critical care, where knowing a patient’s water compartment size directly changes the treatment plan. Athletes and dietitians also use it in a lighter-weight way, as a rough marker of hydration status and lean body mass alongside other measurements.

Age, height, weight and sex go in; your estimated total body water comes out in litres and as a percentage of body weight, using the Watson formula.

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