What this calculator does
Water makes up more of the body than anything else, and the Watson equations estimate how much from age, height, weight and sex. The result is used in dialysis prescribing, in drug distribution calculations and in fluid management.
The share falls steadily with age. A 30 year old 175 cm man at 75 kg is estimated at 44.6 litres, about 59.5 per cent of body weight, dropping to 41.0 litres and 54.6 per cent by 60.
The formula
Separate fitted equations for men and women combine height, weight and, for men, age. The result is in litres, which is also roughly kilograms of water.
| Term | Meaning |
|---|---|
| Total body water | All water in the body, inside and outside cells. |
| Watson formula | The 1980 regression used here, fitted against isotope dilution measurements. |
| Distribution volume | The space a water-soluble drug spreads through, which body water approximates. |
The inputs explained
| Field | What to enter |
|---|---|
| Age (years) | Age in years. It appears in the male equation only. |
| Height (cm) | Height in centimetres. |
| Weight (kg) | Body weight in kilograms. |
| Sex | Sex, which selects between the two equations. |
When to use it
Estimating drug distribution
Water-soluble drugs distribute through body water, which affects loading doses.
Dialysis prescribing
Adequacy measures in dialysis are indexed against total body water.
Understanding alcohol distribution
Alcohol spreads through body water, which is why the same drink affects people differently.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does age affect the estimate?
The same body at three ages.
| Age | Total body water | As a share of body weight |
|---|---|---|
| 20 years | 44.6 L | 59.5% |
| 40 years | 42.8 L | 57.1% |
| 60 years | 41.0 L | 54.6% |
Questions
Why does body water fall with age?
Because lean tissue holds roughly three times as much water as fat, and the proportion of lean tissue tends to decline with age. The same body weight therefore contains less water at 60 than at 20.
Why is there no age term for women?
Because the Watson regression for women did not find age added predictive value once height and weight were accounted for. It is a feature of how the equations were fitted rather than a claim about physiology.
How accurate is it?
It was fitted against isotope dilution measurements and performs reasonably at typical body sizes, but it is less reliable at the extremes and in oedema, dehydration or obesity, where the assumed relationship between weight and water breaks down.
Is this how much water I should drink?
No, and the two are unrelated. This estimates the water already in your body, not intake. Daily fluid needs depend on activity, climate, diet and health, and are a separate question.
For the fat-free mass that holds most of that water, see the lean body mass calculator. For alcohol distributing through it, see the blood alcohol content calculator.