ReadyCalculator
MathPhysicsFinanceConstructionBiologyChemistryConversionEcologyEveryday LifeFoodHealthSportsStatisticsOther

Total Body Water Estimator

Choose an estimation method, enter patient demographics, and calculate total body water to support sodium correction, drug dosing, and dialysis planning.

Used directly in Watson formula; guides fraction adjustment in simplified method.

Total body water

43.9 L

TBW fraction

54.8%

Watson formula accounts for age, height, and weight, offering refined TBW estimates useful for hyponatraemia management, pharmacokinetics, and dialysis dosing.

How to Use This Calculator

1

Select estimation method

Use Watson formula when height and age are available for greater precision. Choose simplified fraction for quick estimates or limited data.

2

Enter patient data

Provide sex, age, weight, and height (Watson only). Ensure units are consistent: height in centimetres, weight in kilograms.

3

Apply TBW clinically

Use TBW to calculate sodium deficit, free water deficit, drug distribution volumes, or dialysis prescriptions. Reassess as patient weight changes.

Formula

Watson (male) = 2.447 − 0.09516 × Age + 0.1074 × Height(cm) + 0.3362 × Weight(kg)

Watson (female) = −2.097 + 0.1069 × Height(cm) + 0.2466 × Weight(kg)

Simplified TBW = Weight(kg) × Fraction (0.6 male, 0.5 female; adjust to 0.5/0.45 for older adults)

Full Description

Total body water estimation underpins fluid and electrolyte calculations, including sodium correction, free water deficit, and drug dosing. The Watson formula remains widely validated, whereas simplified fractions offer quick approximations. Consider patient age, body composition, and disease states (e.g., cirrhosis, CHF) when choosing a method and interpreting results.

Frequently Asked Questions

When should I prefer the Watson formula?

Use Watson whenever height and age are known; it offers better accuracy for pharmacokinetics and hyponatraemia management.

How do obesity and cachexia affect TBW?

Obesity increases fat mass with lower water content, so Watson may overestimate TBW. Consider adjusted body weight or clinical judgment.

Why does TBW fraction change with age?

Older adults have lower muscle mass and higher fat proportion, reducing water content. Fractions of 0.45–0.5 are more appropriate.

Can I use this for paediatrics?

Pediatric TBW differs by age. Use age-specific pediatric formulas rather than Watson or adult fractions.