This total body water calculator estimates how much of you is water — total body water (TBW) in liters and as a share of body weight — using the Watson equations, the standard anthropometric formulas used in clinical and dialysis practice.
Watson equations — men: TBW = 2.447 − 0.09156×age + 0.1074×height + 0.3362×weight; women: TBW = −2.097 + 0.1069×height + 0.2466×weight (the female equation carries no age term in the original fit). Body-composition methods (BIA scales, DXA) estimate the same quantity by measurement instead of regression.
Mostly Water, Precisely Estimated
Lean tissue is ~73% water and fat tissue ~10%, so body water tracks lean mass — which is what the Watson formulas exploit (height and weight proxy the lean frame; the male age term captures its slow decline). The classic splits: ~⅔ intracellular, ~⅓ extracellular (itself split between interstitial fluid and plasma). In medicine, TBW is the volume term in dialysis dosing (Kt/V), drug distribution volumes and fluid-resuscitation math; in body composition it’s the bridge between our Lean Body Mass Calculator‘s output and scale-weight reality. Part of our Health Calculators collection.
Worked Examples (Verified)
- Man, 30, 180 cm, 80 kg: TBW = 2.447 − 2.75 + 19.33 + 26.90 = 45.9 liters = 57.4% of body weight — about 30.6 L inside cells, 15.3 L outside
- Woman, 165 cm, 60 kg: TBW = −2.10 + 17.64 + 14.80 = 30.3 liters = 50.6% — the lower percentage reflects typically higher essential fat share, not “less hydration”
- Rule-of-thumb cross-check: the folk figures (men ~60%, women ~50%, infants ~75%, declining with age and adiposity) bracket Watson’s outputs — when an estimate lands far outside those bands for its inputs, recheck the arithmetic, not the person
Where Estimates Meet Measurement
- BIA scales infer TBW from electrical resistance (water conducts; fat resists) — same target, noisier day-to-day (hydration status moves the reading it claims to measure).
- Dilution methods (deuterium oxide) are the research gold standard Watson was fitted against.
- TBW swings acutely with illness, dialysis, endurance events and refeeding — formulas describe the steady, healthy state.
Related Tools
- Lean Body Mass Calculator — the tissue the water lives in
- Body Surface Area Calculator — the other great anthropometric estimate
- Health Calculators — the full collection
Frequently Asked Questions
How much of the body is water?
Roughly 50–65% for adults: the Watson equations give 57.4% for a 30-year-old 80 kg man and 50.6% for a 60 kg woman in the worked examples. Infants run ~75%; the share falls with age and higher body fat.
What is the Watson formula?
A pair of regressions (Watson, 1980) fitted against isotope-dilution measurements: men use age, height and weight; women use height and weight. They remain the standard estimate in dialysis and nutrition practice.
What is the difference between TBW and daily water intake?
TBW is the standing volume (tens of liters, the reservoir); intake (~2–3.7 L/day from all sources per common guidance) is the daily throughput replacing losses. This calculator measures the reservoir.
Does more muscle mean more body water?
Yes — muscle is water-rich (~73%) and fat is water-poor (~10%), so at equal weight the leaner body holds more liters and a higher percentage. That’s precisely why weight-based formulas include height (a frame proxy) and why athletes out-read sedentary estimates.
How accurate is the estimate?
Against dilution measurements, Watson typically lands within ~±2–3 liters for people near the reference populations — excellent for planning, not a substitute for measured values in acute care.
Is this total body water calculator free?
Yes — free, instant, no sign-up.