How much of you is water
Water is the largest component of the human body: about 60% of body weight in adult men and 50 to 55% in adult women. The share is higher in babies (about 75%) and lower in older adults. Fat tissue holds little water, while muscle is about 75% water, so people with more body fat have a lower share of water.
The Watson formula
In 1980, P. E. Watson and colleagues published formulas that estimate total body water (TBW) in liters from sex, age, height and weight, based on dilution measurements in hundreds of adults:
- Men: TBW = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × weight (kg)
- Women: TBW = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg)
The Watson formula is used in medicine, for example to estimate how alcohol distributes in the body or to plan dialysis doses.
Worked example
A 40-year-old man, 5 ft 9 in (175 cm) and 180 lb (81.6 kg):
- TBW = 2.447 − 0.09516 × 40 + 0.1074 × 175.3 + 0.3362 × 81.6.
- TBW = 2.447 − 3.81 + 18.82 + 27.45 = 44.9 liters.
- Share of body weight: 44.9 ÷ 81.6 = 55%.
- Inside cells: about 29.9 L; outside cells: about 15.0 L.
- The quick 60% rule would give 49 L, slightly higher because it ignores body composition.
Where the water is
| Compartment | Share of total body water | Example (45 L) |
|---|---|---|
| Intracellular (inside cells) | about 2/3 | 30 L |
| Extracellular (outside cells) | about 1/3 | 15 L |
| including interstitial fluid | about 3/4 of extracellular | 11 L |
| including blood plasma | about 1/4 of extracellular | 4 L |
Why it matters
Total body water helps doctors estimate:
- Water deficit in dehydration or high blood sodium.
- Drug and alcohol distribution: substances that dissolve in water spread through this volume, which is why the same drink raises blood alcohol more in a smaller person or one with less muscle.
- Dialysis adequacy, using the Kt/V index.
Limits of the estimate
The Watson formula was built in healthy adults. It is less accurate with obesity, edema, pregnancy, severe illness and in children, for whom other formulas exist. Bioimpedance scales give a rough measurement; dilution methods are the reference.
Daily fluid needs are a separate question: see the water intake calculator for how much to drink.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.