Article — Body Water Calculator
Body Water Calculator: Watson Formula and Hydration Reference
A body water calculator estimates total body water (TBW) using the Watson formula. A typical adult man holds about 60 percent of his weight as water, a woman about 55 percent, and an older adult closer to 50 percent. For a 75 kg man aged 35 with height 175 cm, Watson predicts roughly 44 liters of TBW.
The calculator on this page applies the Watson 1980 regression to your age, sex, height, and weight, then cross-checks the result against the simpler adult rule-of-thumb estimate. Both are population averages, not lab measurements. Deuterium dilution remains the clinical gold standard, but it is rarely run outside research settings.
What is total body water?
Total body water is every liter of water inside cells, between cells, and inside the vascular system. About two thirds sits inside cells as intracellular fluid. The remaining third is extracellular: interstitial fluid, plasma, lymph, cerebrospinal fluid, and a small set of transcellular pools like joint and ocular fluid.
Water is the matrix for nearly every biochemical reaction. It cushions joints, regulates core temperature through sweat, transports oxygen and nutrients in plasma, and removes nitrogenous waste through urine. A drop of just 2 percent in TBW already affects cognitive performance and exercise endurance.
Lean muscle is roughly 75 percent water; adipose tissue is closer to 20 percent. Two people at the same weight can differ by 10 liters of TBW based on muscle versus fat alone.
The Watson body water formula
The Watson equation is a sex-specific regression published in 1980 from deuterium dilution data. The male version reads TBW = 2.447 minus 0.09156 times age plus 0.1074 times height in centimeters plus 0.3362 times weight in kilograms. The female version drops age and reads TBW = negative 2.097 plus 0.1069 times height plus 0.2466 times weight.
The coefficients matter. Weight contributes about a third of a liter of water per kilogram in men but only a quarter in women, reflecting the body composition gap. Age subtracts roughly 0.09 liters per year in men, capturing the slow loss of muscle and rise of body fat across adulthood.
Men 2.447 - 0.09156·age + 0.1074·cm + 0.3362·kgWomen -2.097 + 0.1069·cm + 0.2466·kgAdult rule of thumb M 0.60 × weight (kg)Adult rule of thumb F 0.50 × weight (kg)Body water percent by sex and age
Body water percentage is TBW divided by weight, times 100. It is the friendlier number to interpret because it normalizes across body size.
- Newborns 75 to 80 percent water
- Children 1 to 12 65 to 70 percent
- Adult men 50 to 65 percent (typical ~60)
- Adult women 45 to 60 percent (typical ~55)
- Adults 65+ 45 to 55 percent
- Obese adults 40 to 50 percent due to higher fat fraction
Athletes carry an extra liter or two compared to sedentary peers of the same height and weight, because their lean mass is higher. Endurance training raises plasma volume by 5 to 15 percent within weeks, which is part of why a fit 35 year old runner reports more TBW than the formulas alone predict.
How to use the body water calculator
Enter your age in years, select male or female, then type your weight and height. The widget supports both metric and imperial units. The headline shows your Watson TBW in liters, plus the percent of body weight. A grid below restates the result in fluid ounces, US cups, and gallons.
The Hume-Weyers number is a sanity check. If the two figures disagree by more than five liters, either your body composition is unusually lean or unusually fatty, or one input is wrong.
Weigh yourself in the morning after using the bathroom, before eating or drinking. Body weight can swing 1 to 2 kg across a day from food, sweat, and urine.
Body water vs. daily hydration
TBW is the pool. Daily hydration is the flow in and out of that pool. The Watson formula does not tell you how much to drink; it tells you how much water your body holds at steady state. The National Academies of Sciences, Engineering, and Medicine recommend total daily water intake of about 3.7 liters for adult men and 2.7 liters for adult women, including water from food.
A practical rule is 30 to 35 mL of fluid per kilogram of body weight per day. Hot weather, fever, heavy exercise, and high-protein diets push the requirement up. Kidney disease, congestive heart failure, and certain medications push it down, sometimes drastically.
Clinical uses of body water estimates
Nephrologists use TBW to dose hemodialysis prescriptions. Anesthesiologists use it to estimate volume of distribution for drugs, especially water-soluble agents like aminoglycosides. Pharmacokinetic models lean heavily on TBW because a drug that distributes into total body water has a predictable concentration once dose and TBW are known.
Burn units track TBW closely; the Parkland formula for resuscitation fluid is anchored to body weight, but clinicians adjust for estimated water deficit. In sports medicine, repeated TBW estimates flag chronic underhydration in endurance athletes before it shows as cramping or kidney strain.
Drinking far past thirst, especially during long endurance events, can dilute blood sodium to dangerous levels. More water is not always better. Match intake to sweat losses, not to a flat daily quota.
Common body water mistakes
The first mistake is confusing TBW with drinking targets. Pouring extra liters down the hatch does not raise TBW past its equilibrium; the kidneys clear the excess within an hour. The second is trusting a smart scale that prints body water percent. Bioelectrical impedance is noisy, shifts with the time of day, and varies by 3 to 5 percentage points within the same person across a week.
The third is ignoring medication effects. Diuretics, lithium, and certain anticonvulsants change TBW. Steroids and some antidepressants can drive fluid retention. If a formula result disagrees with how you feel, the medication list is the first place to look.