Water makes up about half to two-thirds of body weight. Muscle holds much more water than fat, so lean people have a higher percentage. Total body water is used for drug dosing, dialysis prescriptions and sodium and water calculations.
How to use the Total Body Water Calculator
- Sex
- Age
- Height
- Weight
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter sex, age, height and weight.
Inputs explained
| Field | What to enter |
|---|---|
| Sex | Choose one: Male, Female |
| Age | In years |
| Height | Units: cm, in, m, ft, mm |
| Weight | Units: kg, lb, st, g |
What each result means
- Water as % of body weight = [Total body water (Watson)]÷[Weight]×100
- Intracellular water (about 2/3) = [Total body water (Watson)]×2÷3
- Extracellular water (about 1/3) = [Total body water (Watson)]÷3
Example values
How the main result changes when Age changes and everything else stays at the example values above:
| Age (years) | Total body water (Watson) (L) |
|---|---|
| 20 | 44.6 |
| 30 | 43.6 |
| 40 | 42.7 |
| 50 | 41.7 |
| 60 | 40.7 |
| 80 | 38.8 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Watson – men: TBW = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × weight (kg)
Watson – women: TBW = −2.097 + 0.1069 × height + 0.2466 × weight
Simple: 60% (men) or 50% (women) of weight
Worked example
Inputs — Sex: Male; Age: 40 years; Height: 175 cm; Weight: 75 kg.
Results — Total body water (Watson) = 42.7 L; Water as % of body weight = 57 %; Simple estimate = 45 L; Intracellular water (about 2/3) = 28.4 L; Extracellular water (about 1/3) = 14.2 L.
Key terms
- TBW
- Total body water.
- ICF/ECF
- Intracellular and extracellular fluid.
Frequently asked questions
What percentage of the body is water?
About 50–65% in adults, more in infants.
Why do women have less body water?
They usually have more body fat, which holds little water.
How does age affect body water?
It falls as muscle mass declines.