The Watson formula, split by sex
TBW is in liters, height in cm, weight in kg, age in years. Watson is the standard clinical estimate of total body water — all the fluid inside and around your cells. A quick sanity check: it usually lands near 60% of body weight for men and about 50% for women.
Typical water content by group
| Group | % of body weight | At 70 kg |
|---|---|---|
| Adult men | 55–65% | 38–45 L |
| Adult women | 45–60% | 31–42 L |
| Older adults (65+) | 45–55% | 31–38 L |
| Children | 65–70% | — |
Women and older adults sit lower because they carry a larger fat fraction, and fat holds far less water than muscle.
What moves the number
- Muscle vs fat. Muscle is about 75% water, fat only around 20%, so body composition drives the percentage more than total weight does.
- Day-to-day swing. Meals, drinking, sweat and urine shift body water by 1 to 2 kg across a single day, so a formula gives a baseline, not a fixed value.
- Age. Water content drops with age as muscle is replaced by fat — the reason the mens equation subtracts a term for age.
Common questions
What percentage of my body is water?
Roughly 55 to 65% for adult men and 45 to 60% for adult women. Muscle holds far more water than fat, so a leaner body sits at the higher end and body water falls with age.
Why does the mens formula use age but the womens does not?
The Watson equation was fitted separately to each sex. For men, water tracks measurably down with age, so age carries a negative term; for women the age effect was too small to keep, so the formula uses only height and weight.
Is the Watson estimate accurate for everyone?
It is most reliable for adults with a normal BMI. It tends to over- or under-estimate for people with obesity or very high muscle mass, because it assumes a typical body composition.
