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Estimate body water percentage from weight, height, age and sex using Watson formula — see total body water liters and percent.
Estimate body water percentage from weight, height, age and sex using Watson formula — see total body water liters and percent.
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Total body water is predicted from weight, height, age and sex through the Watson equations derived from isotope dilution studies: male total water equals 2.447 minus 0.09516 times age plus 0.1074 times height plus 0.3362 times weight, with a parallel female formula. Dividing predicted water in liters by weight and multiplying by 100 yields percent — typically 50–65% in healthy adults, trending lower with higher adiposity and with age as muscle mass declines, and generally a few points lower in women due to average fat fraction.
Watson is an anthropometric estimate, not bioimpedance or isotope measurement; dehydration, edema, diuretic use and altered body composition all degrade its accuracy for individuals. Use the percent to contextualize hydration targets and weight changes, not to diagnose fluid status — clinical fluid assessment relies on examination and labs, not equations.
Where body water lives matters: roughly two-thirds intracellular (inside cells, tied to muscle and glycogen — each gram of glycogen stores ~3 g water, explaining rapid 'water weight' swings on diet changes) and one-third extracellular (plasma plus interstitial fluid). Creatine loading, carb refeeds and menstrual cycles shift liters without any fat change — scale-weight jumps after such shifts are water, not tissue.
Age and sex patterns are systematic: newborns are ~75% water, young adult men ~60–65%, women ~50–55% (higher essential fat), and both sexes drift down with sarcopenia — an 80-year-old may sit near 50% and 45% respectively. Athletes run higher via muscle mass; obesity lowers percent because adipose holds little water (~10%) versus muscle (~75%).
Clinical uses of TBW estimates (outside this educational tool): drug dosing for hydrophilic drugs (aminoglycosides, lithium) scales with lean/water compartments, dialysis dry-weight targets reference it, and fluid-resuscitation formulas start from TBW fractions. All clinical applications use measured or validated methods plus judgment — equations screen, they never prescribe.
Hydration guidance from your number: daily water needs run roughly 30–35 ml per kg (higher in heat, exercise, illness, breastfeeding), urine should stay pale-yellow, and thirst lags losses by 1–2% body weight in heat. A falling TBW percent alongside stable weight suggests muscle loss; alongside falling weight, it confirms fat-plus-water loss typical of deficits.
Estimate body water percentage from weight, height, age and sex using Watson formula — see total body water liters and percent. Formula: Watson: Male TBW =2.447 -0.09516*age +0.1074*height +0.3362*weight. Example: At 70kg, 175cm, 30 male: TBW about 42.1L, ~60.1% of weight.
Total body water is predicted from weight, height, age and sex through the Watson equations derived from isotope dilution studies: male total water equals 2.447 minus 0.09516 times age plus 0.1074 times height plus 0.3362 times weight, with a parallel female formula. Dividing predicted water in liters by weight and multiplying by 100 yields percent — typically 50–65% in healthy adults, trending lower with higher adiposity and with age as muscle mass declines, and generally a few points lower in women due to average fat fraction.
Watson is an anthropometric estimate, not bioimpedance or isotope measurement; dehydration, edema, diuretic use and altered body composition all degrade its accuracy for individuals. Use the percent to contextualize hydration targets and weight changes, not to diagnose fluid status — clinical fluid assessment relies on examination and labs, not equations.
Where body water lives matters: roughly two-thirds intracellular (inside cells, tied to muscle and glycogen — each gram of glycogen stores ~3 g water, explaining rapid 'water weight' swings on diet changes) and one-third extracellular (plasma plus interstitial fluid). Creatine loading, carb refeeds and menstrual cycles shift liters without any fat change — scale-weight jumps after such shifts are water, not tissue.
Age and sex patterns are systematic: newborns are ~75% water, young adult men ~60–65%, women ~50–55% (higher essential fat), and both sexes drift down with sarcopenia — an 80-year-old may sit near 50% and 45% respectively. Athletes run higher via muscle mass; obesity lowers percent because adipose holds little water (~10%) versus muscle (~75%).
Clinical uses of TBW estimates (outside this educational tool): drug dosing for hydrophilic drugs (aminoglycosides, lithium) scales with lean/water compartments, dialysis dry-weight targets reference it, and fluid-resuscitation formulas start from TBW fractions. All clinical applications use measured or validated methods plus judgment — equations screen, they never prescribe.
Hydration guidance from your number: daily water needs run roughly 30–35 ml per kg (higher in heat, exercise, illness, breastfeeding), urine should stay pale-yellow, and thirst lags losses by 1–2% body weight in heat. A falling TBW percent alongside stable weight suggests muscle loss; alongside falling weight, it confirms fat-plus-water loss typical of deficits.
At 70kg, 175cm, 30 male: TBW about 42.1L, ~60.1% of weight. After a carb-depleted week the same person might read 40.5L — the 1.6L gap is glycogen-water, not fat, and returns with refeeding.
Formulas are standard public references (see our methodology). External standards are cited in the text where they apply.
Last reviewed: September 2026 · Report an error