π§ Body Water Calculator
Total body water from the Watson formula, in liters and percent.
Estimates for planning only, not medical advice.
About 60 percent of you is water, and clinicians have a formula for exactly how much. The Watson formula, published in 1980 and still the clinical standard, estimates total body water from age, sex, height, and weight. This calculator runs it and shows your TBW in liters plus what share of your body weight that represents, which is the number hydration discussions actually revolve around.
How to Use This Calculator
Enter sex, age, height in centimeters, and weight in kilograms. The result appears instantly.
Enter your stats. Use morning weight for consistency.
Read total body water in liters and as a percentage of body weight.
Compare against the typical range: 55 to 65 percent for men, 45 to 60 percent for women. Athletes run higher, older adults lower.
Turn the number into a daily drinking target with our water intake calculator.
The Watson Formula
Watson, Watson, and Butt derived these equations in the American Journal of Clinical Nutrition from dilution studies, the gold-standard method where a tracer is drunk and its dilution measured. Water lives mostly in lean tissue: muscle is about 75 percent water while fat is only about 10 percent, which is why the formula gives men more water than women at the same weight and why age subtracts water in men (muscle declines with age). The female equation skips the age term because the original data fit better without it.
Clinicians use TBW constantly: drug dosing for water-soluble drugs, dialysis prescriptions, and fluid management all scale with it. For the rest of us it is mostly a hydration reality check. If your smart scale reports body water percentage, compare it against this formula's estimate; when the two disagree by more than a few points, trust Watson, because BIA scales swing with every glass of water you drink.
Worked Example
A 35-year-old man, 178 cm, 82 kg. TBW = 2.447 - 0.09156 x 35 + 0.1074 x 178 + 0.3362 x 82 = 2.447 - 3.205 + 19.117 + 27.568 = 45.9 liters. As a share of weight: 45.9 / 82 = 56.0 percent, inside the typical 55 to 65 percent band for men. A 35-year-old woman at 165 cm and 65 kg: TBW = -2.097 + 0.1069 x 165 + 0.2466 x 65 = -2.097 + 17.639 + 16.029 = 31.6 liters, or 48.6 percent, inside the typical 45 to 60 percent band for women. Same age, different composition, different water.
What Moves Your Water Percentage
Muscle is the biggest lever: gaining 5 kg of muscle adds roughly 3.7 liters of water to the estimate, which is why athletes read high. Fat is the anchor: it adds weight with almost no water, so higher body fat always means a lower water percentage. Age drags it down as muscle gives way to fat, and even dehydration barely moves the formula's estimate, because TBW here is a structural number, not a today-I-drank-enough number. Real dehydration shows up in urine color and thirst long before it shows up here.
That last point matters for interpretation. This calculator estimates your body's water compartment size, not your current hydration status. You cannot meaningfully change the percentage by chugging water this afternoon; you change it over months by changing body composition. If the number reads low, the fix is more muscle and less fat, tracked with a body fat reading, not a bigger water bottle. Though drinking enough water remains good advice regardless, and our intake calculator personalizes the daily target.
TBW in Medicine: Why Doctors Care
Beyond curiosity, total body water sets dosing for drugs that distribute in body water, from alcohol to lithium to many antibiotics. It explains why the same drink hits a smaller person harder: less water means higher blood alcohol concentration, which is also why women reach higher BAC than men at the same intake. Dialysis adequacy (Kt/V) is normalized to TBW. None of this makes the number actionable at home, but it explains why a 1980 formula still appears in every nephrology textbook: water is the body's main compartment, and everything dissolves in it.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is a normal body water percentage?
Roughly 55 to 65 percent of body weight for adult men and 45 to 60 percent for women. Athletes run higher because muscle holds water; older adults and people with higher body fat run lower. This calculator estimates yours with the clinical Watson formula.
What is the Watson formula for total body water?
For men: TBW = 2.447 - 0.09156 x age + 0.1074 x height(cm) + 0.3362 x weight(kg). For women: TBW = -2.097 + 0.1069 x height + 0.2466 x weight. Published by Watson et al. in 1980 from tracer-dilution studies, it remains the clinical standard.
Why do men have a higher body water percentage than women?
Muscle is about 75 percent water and fat only about 10 percent, and men carry more muscle on average at the same weight. That composition difference, not size alone, drives the gap.
Does drinking more water raise my body water percentage?
Not meaningfully. This percentage reflects body composition, which changes over months, not hydration today. Drinking water is still important for how you feel and function, but it will not move this number. Build muscle and lose fat to move it.
How accurate is the Watson formula?
It estimates the water compartment within a few percent for typical adults. It is less accurate at extremes of obesity, where the underlying data were thin, and it estimates structural water, not day-to-day hydration swings.
Why does body water percentage drop with age?
Muscle mass declines with age while fat mass tends to rise, and since muscle holds far more water than fat, total body water falls as a share of weight. Strength training is the direct countermeasure.
My smart scale shows a different water percentage. Which is right?
Trust the Watson estimate over the scale. Smart scales use bioelectrical impedance, which is heavily influenced by recent drinking, meals, and skin moisture, swinging several points day to day. The formula gives the stable structural number.
How much water is in the human body in liters?
For a typical adult man around 40 to 50 liters, for a typical adult woman around 30 to 38 liters. Enter your stats above for your personal estimate from the Watson formula.
Is total body water used in medicine?
Constantly. It scales dosing for water-soluble drugs, sets dialysis prescriptions, guides fluid resuscitation, and explains alcohol sensitivity differences by size and sex. It is one of the most-used physiological estimates in clinical practice.
Can dehydration be measured by body water percentage?
Not with this formula, which estimates your stable water compartment. Acute dehydration is assessed by symptoms: thirst, dark urine, dizziness, and reduced urine output. If those appear, drink fluids; the percentage on this page will not tell you much.