Total body water from age, height, weight and sex — the same equation nephrologists use to calculate haemodialysis Kt/V. Informational reference only, not a medical test.
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Typical adult range (55–65 %)
41.0L total body water
Total body water: 41.0 L
As % of weight: 58.6 %
Typical adult range (men): 55–65 %
Watson formula (male) — clinical estimate, not BIA-measured.
Watson’s prediction equations (1980) are the most widely used clinical estimate of total body water — they’re embedded in haemodialysis adequacy calculations (Kt/V). Typical adult ranges are 55–65 % of body weight for men, 45–60 % for women. Higher body-fat percentages reduce TBW % (fat tissue is ~10 % water, lean tissue ~75 %). For accurate measurement, bioelectrical impedance analysis (BIA) scales like Withings or Tanita are within ±3 % of DEXA. Informational only — not a medical diagnostic.
What this number tells you
Total body water (TBW) is the sum of intracellular and extracellular fluid — typically about 60 % of an adult’s mass for men and 50 % for women. The Watson formula is the most-cited clinical estimate, derived from a 1980 multi-centre study and validated against deuterium-dilution measurements.
What changes body water
Body water % goes down with: increasing body-fat percentage, dehydration, ageing past 60, and significant alcohol intake. Body water % goes up with: increased lean muscle mass, regular hydration, and being female of childbearing age in luteal phase (slight upward shift from progesterone). These daily fluctuations are typically ±1–2 percentage points around your trend.
Limitations
Watson’s formula assumes a typical adult body composition. It under-estimates TBW for very lean / muscular bodies and over-estimates for very high body-fat compositions. It’s also been validated only in adults — for children, the Mellits-Cheek 1970 paediatric equations are more accurate. For clinical applications (dialysis, drug dosing) physicians use the formula in combination with BIA and clinical judgement, not as a stand-alone number.
This tool is for educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a licensed Canadian healthcare professional. Read our full disclaimer.