Body Water Percentage Calculator

The Watson formula (Watson PE et al., Am J Clin Nutr. 1980;33:27–39) estimates total body water (TBW) in liters from simple measurements. For men: TBW = 2.447 − 0.09156 × age (years) + 0.1074 × height (cm) + 0.3362 × weight (kg). For women: TBW = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg). It was derived from dilution studies in 458 adult men and 265 adult women, and it is still widely used clinically, for example to estimate the urea distribution volume in dialysis.

Divide the Watson total body water (in liters, numerically equal to kilograms of water) by body weight in kilograms and multiply by 100. Example: a 40-year-old man, 180 cm, 80 kg → TBW ≈ 45.0 L, so 45.0 ÷ 80 × 100 ≈ 56.3%. Example: a 30-year-old woman, 165 cm, 60 kg → TBW ≈ 30.3 L, so 30.3 ÷ 60 × 100 ≈ 50.6%. The calculator above does this arithmetic for you and shows the simple 0.6/0.5 method alongside for comparison.

Educational context only, not a diagnosis. Broadly, adult men average around 55–65% (often quoted as ~60%) and adult women around 45–55% (often quoted as ~50%), with the percentage drifting down with age as muscle mass declines. One published reference table gives: men 19–50 years average 59% (range 43–73%), men 51+ average 56% (47–67%); women 19–50 average 50% (41–60%), women 51+ average 47% (39–57%). Athletes tend to read higher and people with more body fat tend to read lower, because fat tissue holds little water.

Different question entirely. This tool estimates your body water PERCENTAGE — how much of your weight is water — using the Watson formula (age, height, weight). The water-deficit calculator estimates the free-water DEFICIT in hypernatremia with the Adrogue–Madias equation: deficit = TBW × (Na ÷ 140 − 1). Use this page for body-composition context; use the water-deficit calculator for sodium/electrolyte planning. They are linked below as related tools.

That is how Watson’s best-fit regression came out of the data: for women, adding age did not improve the prediction enough to keep it in the final equation, while for men the age term (−0.09156 per year) was significant — reflecting the steady loss of muscle (and its water) with aging. In practice this means a woman’s Watson estimate is driven by height and weight only.

The simple method (0.6 × kg for men, 0.5 × kg for women) is a quick bedside rule of thumb. Watson refines it with age and height: for example, an 80 kg, 40-year-old, 180 cm man gives 48 L by the simple method but ≈ 45.0 L by Watson. Watson is generally the better estimate for adults of typical build, while the simple method is fine for rough mental math. Both break down in obesity, the very elderly, children, and critical illness.

No — with the usual caveats. Watson’s equations were derived from adults, so they are not validated for children. In obesity they overestimate, because excess fat adds weight without proportional water; in very muscular athletes they underestimate the percentage for the opposite reason. Treat every result here as an educational estimate, not a body-composition measurement.

No. This tool is for education and study only — it is not medical advice, it does not diagnose any condition, and it must not guide fluid therapy, dialysis dosing, or any clinical decision. If you have concerns about hydration, kidney function, or body composition, consult a qualified clinician.

Metric or imperial: weight in kilograms or pounds, height in centimeters or feet+inches, age in years. Results are always shown in liters (TBW) and percent of body weight, with the simple-method comparison alongside.

No. This calculator runs 100% in your browser with zero network requests — your age, height, and weight never leave your device.