Total Body Water Calculator
The Watson equations come from about a thousand people whose body water was actually measured by deuterium dilution. A 35-year-old man of 175 cm and 70 kg comes out at 41.57 L, which is 59.4% of body weight. Lean tissue is 73.2% water, so dividing body water by 0.732 also estimates lean mass — 56.8 kg here. The equation for women carries no age term at all, so changing age leaves their figure untouched.
Your numbers
Total body water
41.57L
Water as share of weight
59.4%
Lean mass
56.79kg
Formula
Total body water = 2.447 − 0.09156 × Age + 0.1074 × Height (cm) + 0.3362 × Weight (kg)
A regression describes the average of a population, so individuals scatter above and below it. Body water also moves within a single day: salty food, a sweaty session or a carbohydrate load can put the real figure litres away from this estimate.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Sex (M1 / F0)Where the formula splits by sex — enter 1 for male, 0 for female. | 1 | 0 ~ 1 |
| Age (yr)Age in completed years — it shifts by one depending on the birthday. | 35 | 0 ~ 100 |
| Height (cm) (cm)Height in centimetres; converted to metres inside the formula. | 175 | 0 ~ 250 |
| Weight (kg) (kg)The second weight you are comparing against. | 70 | 0 ~ 250 |
Step by step
Quick reference table
Results when only Age (yr) changes and everything else stays put.
| Age (yr) | Total body water (L) | Water as share of weight (%) | Lean mass (kg) |
|---|---|---|---|
| 17.5 | 43.17 | 61.7 | 58.98 |
| 26.3 | 42.37 | 60.5 | 57.88 |
| 35 | 41.57 | 59.4 | 56.79 |
| 52.5 | 39.97 | 57.1 | 54.6 |
| 70 | 38.37 | 54.8 | 52.41 |
What each result means
| Result | At default values |
|---|---|
| Total body water (L)All the water in the body, inside and outside the cells, added together. | 41.57 |
| Water as share of weight (%)That water as a share of body weight; more fat pushes it down. | 59.4 |
| Lean mass (kg)Everything that is not fat — muscle, bone, water. | 56.79 |
Common mistakes
A regression describes the average of a population, so individuals scatter above and below it. Body water also moves within a single day: salty food, a sweaty session or a carbohydrate load can put the real figure litres away from this estimate.
Glossary
- Sex (M1 / F0)
- Where the formula splits by sex — enter 1 for male, 0 for female.
- Age
- Age in completed years — it shifts by one depending on the birthday.
- Height (cm)
- Height in centimetres; converted to metres inside the formula.
- Weight (kg)
- The second weight you are comparing against.
- Total body water
- All the water in the body, inside and outside the cells, added together.
- Water as share of weight
- That water as a share of body weight; more fat pushes it down.
- Lean mass
- Everything that is not fat — muscle, bone, water.
Frequently asked questions
Q. How is Total Body Water Calculator calculated?
Total body water = 2.447 − 0.09156 × Age + 0.1074 × Height (cm) + 0.3362 × Weight (kg) — The Watson equations come from about a thousand people whose body water was actually measured by deuterium dilution. A 35-year-old man of 175 cm and 70 kg comes out at 41.57 L, which is 59.4% of body weight. Lean tissue is 73.2% water, so dividing body water by 0.732 also estimates lean mass — 56.8 kg here. The equation for women carries no age term at all, so changing age leaves their figure untouched.
Q. Can you walk through an example?
With Sex (M1 / F0) 1, Age 35yr, Height (cm) 175cm, Weight (kg) 70kg, the answer is Total body water 41.57L.
Q. What do I need to enter?
Enter Sex (M1 / F0), Age, Height (cm), Weight (kg). The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Age (yr) moves the answer to Age (yr) 17.5 → Total body water (L) 43.17 and Age (yr) 70 → Total body water (L) 38.37. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
A regression describes the average of a population, so individuals scatter above and below it. Body water also moves within a single day: salty food, a sweaty session or a carbohydrate load can put the real figure litres away from this estimate.
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These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.