Body Water Calculator
Estimate how much water your body holds, in litres and as a share of your weight, from your sex, age, height and weight.
Pick any unit for each, for example feet and inches with kilograms.
Adds the lean mass × 0.73 estimate below. Don't know it? Try the body fat calculator.
Other ways to estimate it
| Method | Water (L) | Of weight | vs Watson |
|---|
The 60% / 50% rule and lean mass × 0.73 are rough textbook figures; the inside / outside split is a common rough rule.
How your body water is worked out
Your numbers, step by step, with the Watson formula.
- 1
Age, height, weight
30 y, 175 cm, 75 kg, man
- 2
Watson formula
2.447 − 2.855
+ 18.795 + 25.215
= 43.6 L - 3
Share of your weight
43.6 ÷ 75 kg
= 58.1% of you - 4
Inside and outside
29.1 L inside cells
14.5 L outside
Read more: the formula, a worked example and notes
Total body water (TBW) is all the water your body holds: inside your cells, in your blood and in the fluid around your cells. Doctors use it to work out some fluid and salt corrections and to plan dialysis, and it is a building block for other body measures. Measuring it needs a lab tracer test, so most of the time it is estimated from height, weight, age and sex.
This is an estimate from a formula, not a measurement. Smart scales (bioimpedance) measure in a different way and will often show a different number. General information, not medical advice.
The formula
Watson (1980), weight in kg, height in cm, age in years: men total body water (litres) = 2.447 − 0.09516 × age + 0.1074 × height + 0.3362 × weight; women total body water = −2.097 + 0.1069 × height + 0.2466 × weight. Share of body weight = water ÷ weight × 100. Inside cells ≈ water × 2 ÷ 3; outside cells ≈ water ÷ 3.
The other estimates under Advanced:
| Method | Men | Women | Notes |
|---|---|---|---|
| Hume–Weyers (1971) | 0.194786 × cm + 0.296785 × kg − 14.012934 | 0.34454 × cm + 0.183809 × kg − 35.270121 | older formula, no age term |
| Share of weight rule | 0.6 × kg | 0.5 × kg | rough textbook figure |
| Lean mass × 0.73 | kg × (1 − body fat % ÷ 100) × 0.73 | lean tissue is roughly 73% water; approximate | |
Worked example
A man aged 30, 175 cm tall, weighing 75 kg. Watson: 0.09516 × 30 = 2.8548, 0.1074 × 175 = 18.795 and 0.3362 × 75 = 25.215, so total body water = 2.447 − 2.8548 + 18.795 + 25.215 = 43.6022, shown as 43.6 litres (46.1 US quarts, 76.7 UK pints).
Share of body weight = 43.6022 ÷ 75 × 100 = 58.1%. Inside cells ≈ 43.6022 × 2 ÷ 3 = 29.1 L (29.068); outside cells ≈ 43.6022 ÷ 3 = 14.5 L (14.534).
For the same man, Hume–Weyers gives 0.194786 × 175 + 0.296785 × 75 − 14.012934 = 34.08755 + 22.258875 − 14.012934 = 42.3 L, and the 60% rule gives 0.6 × 75 = 45.0 L. If his body fat were 18%, lean mass = 75 × 0.82 = 61.5 kg, and 61.5 × 0.73 = 44.9 L.
Typical results (Watson)
| Example | Age, height, weight | Body water | Of weight |
|---|---|---|---|
| Woman | 30 y, 160 cm, 55 kg | 28.6 L | 51.9% |
| Woman | 50 y, 168 cm, 75 kg | 34.4 L | 45.8% |
| Man | 30 y, 175 cm, 75 kg | 43.6 L | 58.1% |
| Man | 60 y, 182 cm, 95 kg | 48.2 L | 50.8% |
Notes
- Litres and kilograms: a litre of body water weighs about 1 kg, which is why the share of body weight is simply litres ÷ kg.
- Age: Watson's formula for men loses about 0.1 L per year of age; the women's formula has no age term. Both were built from adults, so results for children are a rough guide only.
- Body fat: fat holds little water, so two people of the same weight can have quite different body water. The formulas only see height and weight, so very muscular or very heavy people can be well off.
- Smart scales: bioimpedance scales estimate body water from how easily a small current passes through you. Their readings move with drinks, meals, exercise and the time of day, so don't expect them to match this page.
- Sources: Watson, Watson and Batt, American Journal of Clinical Nutrition 1980 (33:27–39); Hume and Weyers, Journal of Clinical Pathology 1971 (24:234–238). Coefficients checked on 14 October 2026.
Common questions
What is total body water?
Total body water (TBW) is all the water inside you: in your cells, in your blood and in the fluid around your cells. In adults it is usually about half to two thirds of body weight. Men tend to have a higher share than women because they usually carry more muscle, which holds more water than fat.
How do you calculate total body water?
The Watson formula is the one most used. For a man: 2.447 − 0.09516 × age + 0.1074 × height in cm + 0.3362 × weight in kg. For a woman: −2.097 + 0.1069 × height in cm + 0.2466 × weight in kg. A 30-year-old man, 175 cm and 75 kg, gets 2.447 − 2.8548 + 18.795 + 25.215 = 43.6 litres.
What percentage of my body is water?
Divide your total body water in litres by your weight in kg (a litre of water weighs about a kilogram) and times by 100. Textbooks often say about 60% for men and 50% for women, but it falls with more body fat and with age, so your own figure can be well away from those.
Is this the same as my smart scale body water reading?
No. A smart scale sends a tiny current through you (bioimpedance) and uses its own formula, which changes with how much you have drunk, recent exercise and the time of day. This page uses a formula from height, weight, age and sex. Neither is a true measurement; a lab measures body water with tracer methods such as deuterium dilution.
What are intracellular and extracellular water?
Intracellular water is the water inside your cells; extracellular water is the rest, in blood plasma and the fluid between cells. A common rough rule is two thirds inside and one third outside. The real split varies from person to person.
Does this tell me how much water to drink?
No. Total body water is how much water your body holds, not how much to drink. For a daily drinking estimate, use the water intake calculator. This page is general information, not medical advice.
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