Body Water Percentage Calculator (Watson formula)

Total body water in litres and as % of body mass, via the Watson (1980) anthropometric formula. Illustrative — consult a professional for clinical use.

Inputs

Watson 1980 used two sex-specific equations derived from D₂O-dilution studies.

For lb→kg: divide by 2.2046.

For in→cm: multiply by 2.54.

Result

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How to use this calculator

  • Enter biological sex (the formula has two sex-specific regressions).
  • Age in years.
  • Weight in kilograms (or lb ÷ 2.2046).
  • Height in centimetres (or in × 2.54).
  • Compare your % to the reference band shown for your sex.

About this calculator

Total body water (TBW) is the amount of water in your body — typically 50-65% of body mass in adults, lower in those with higher body fat (fat tissue holds less water than lean tissue). The Watson formula (1980) estimates TBW from sex, age, height, and weight using a regression developed from deuterium-oxide-dilution studies in healthy adults. It is the most widely cited anthropometric TBW estimator in clinical pharmacology (used to dose hydrophilic drugs and to plan dialysis) and in nutrition research. The two sex-specific equations: TBW_male = 2.447 − 0.09156·age + 0.1074·height_cm + 0.3362·weight_kg; TBW_female = −2.097 + 0.1069·height_cm + 0.2466·weight_kg. About two-thirds of TBW is intracellular fluid (inside cells); one-third is extracellular (blood plasma + interstitial fluid). Healthy adult ranges: men 55-65% body water, women 45-60% — women average lower because of higher essential body-fat percentage. Lean athletes can run higher; people with obesity tend to run lower. This calculator is illustrative; it cannot diagnose dehydration or fluid-balance disorders.

Frequently asked

How accurate is the Watson formula?+
Within ±2-3 litres for most healthy adults; less accurate for the very obese, the elderly, or those with edema/ascites. The original Watson paper reported a standard error of ~3.5 L. For clinical use (drug dosing, dialysis prescription) it is the standard estimator; for fitness use it is a rough but useful estimate.
Why are male and female formulas different?+
Body composition differs systematically by sex: women have higher essential body-fat percentage (~12% vs ~3% for elite-fit men; ~25-31% vs 18-24% for typical adults). Since fat tissue holds ~10% water by mass and lean tissue holds ~73%, sex-specific equations are required.
How does this compare to bioelectrical-impedance scales?+
Bioimpedance (BIA) scales measure water directly via tissue resistance; Watson estimates indirectly from anthropometrics. Studies (Heymsfield et al. 2005) show the two agree to ±2-4 L on average, but BIA shifts with hydration timing (post-meal, post-exercise) while Watson does not.
Can I track dehydration with this?+
No — Watson uses only sex/age/height/weight, which don't change with acute dehydration. For day-to-day hydration status, use urine colour, body weight changes, or BIA. Watson gives a stable baseline.
What about children?+
Watson 1980 was developed in adults (18+). Children have higher body water % (~70-75% in infants, declining through childhood). Pediatric estimators (Mellits-Cheek, 1970) should be used for those under 18. This calculator clamps age ≥ 18.
Source?+
Watson PE, Watson ID, Batt RD (1980), "Total body water volumes for adult males and females estimated from simple anthropometric measurements", American Journal of Clinical Nutrition 33: 27-39. Reference ranges per Mitchell et al. (1945) Journal of Biological Chemistry; Heymsfield SB et al. (2005) Human Body Composition, 2nd ed.

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