Last updated October 10, 2026 · Report an error
Estimates total body water (TBW) using the standard published Watson formula (Watson PE, Watson ID, Batt RD, "Total body water volumes for adult males and females estimated from simple anthropometric measurements", American Journal of Clinical Nutrition, 1980), which uses different coefficients for men and women based on age, height, and weight: Men: TBW (L) = 2.447 - (0.09145 x age) + (0.1074 x height cm) + (0.3362 x weight kg). Women: TBW (L) = -2.097 + (0.1069 x height cm) + (0.2466 x weight kg). For general reference and educational purposes only — not a medical diagnosis. Consult a doctor for actual fluid status assessment.
How it works
Enter sex, age, height in cm, and weight in kg. The tool applies the corresponding Watson formula for that sex.
- Enter sex.
- Enter age (years).
- Enter height (cm).
- Enter weight (kg).
- Click Calculate to see your results.
Examples
Male, age 40, height 175 cm, weight 75 kg
TBW = 2.447 - (0.09145 x 40) + (0.1074 x 175) + (0.3362 x 75) ≈ 2.447 - 3.658 + 18.795 + 25.215 ≈ 42.8 L.
Who should use it
- Fluid balance and hyponatremia correction reference calculations.
- Physiology and nephrology education about body water estimation.
Industry applications
- Nephrology and critical care reference
- Clinical pharmacology education
Advantages
- Long-established, widely cited anthropometric formula.
- Simple — needs only routinely available measurements.
Limitations
- An estimate — actual body water varies with body composition beyond age/height/weight.
- Derived from and validated in an adult population only.
Common mistakes to avoid
- Entering height in inches or weight in pounds without converting to cm/kg first (this formula expects metric units).
- Applying the adult Watson formula to a pediatric patient, where it was not derived or validated.
Best practices
- Use current, accurate height and weight measurements in metric units (cm, kg) for the most reliable estimate.
Tips
- TBW estimates are typically one input among several in fluid and electrolyte correction calculations, not used in isolation.