Body Water Percentage Calculator
Total body water from the Watson formula — how many litres you carry and what share of your weight it is.
The Watson equations estimate total body water from height, weight, age and sex.
How the body water percentage calculator works
The Watson equations estimate total body water from height, weight, age and sex. They were derived from deuterium dilution studies and remain the standard field estimate.
The share differs by sex and body composition because muscle is about 75% water and fat only 10 to 20%. A lean man is typically 60% water, a woman 50 to 55%, and both figures fall with age as lean mass declines.
Formula: men: 2.447 − 0.09516×age + 0.1074×height + 0.3362×weight
Worked examples
| Inputs | Total body water | Note |
|---|---|---|
| Male, 35, 175 cm, 75 kg | 43.13 L | about 43 L, 57% |
| Female | 30.83 L | about 31 L, 50% |
| Older | 39.8 L | lower — lean mass declines |
FAQFrequently asked questions
What percentage of the body is water?
About 55 to 65% for adult men and 45 to 60% for women. The difference is body composition, not sex as such.
Why do women carry a lower share?
Because they carry more essential body fat, and fat holds only 10 to 20% water against muscle's 75%.
Does it change with age?
Yes, downward. Lean mass declines with age, and with it the water share — from around 75% in infancy to nearer 50% in old age.
What is the Watson formula?
A regression from deuterium dilution studies, published in 1980, still the standard field estimate for total body water.
Can I change my body water percentage?
Only through body composition. Building muscle and losing fat raises it; drinking more water does not — the body excretes the surplus within hours.
Where these figures come from
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- Australian Department of Health and Aged Care — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.