Calculate total body water with the Watson formula from sex, age, height and weight, and view results in liters or as water weight.
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Disclaimer: This calculator provides an educational estimate and is not medical advice or a diagnostic tool. Do not use it to guide IV fluids, diuretics, or treatment decisions. Talk with a clinician if you are pregnant, under 18, older, or have kidney, heart, or liver disease, edema, or fluid-balance concerns; seek urgent care for severe dehydration symptoms (confusion, fainting, inability to keep fluids down).
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Total Body Water (TBW) Formula
The calculator estimates total body water using the Watson formula. It first converts height to centimeters and body weight to kilograms, then applies the sex-specific equation.
Male Watson formula
Female Watson formula
- TBW = estimated total body water, in liters
- A = age, in years
- H = height, in centimeters
- W = body weight, in kilograms
The calculator also converts the result into other output units when selected.
- L = liters of estimated total body water
- m³ = cubic meters
- ft³ = cubic feet
- kg_water = approximate water weight in kilograms
- lb_water = approximate water weight in pounds
The sex selector chooses the male or female Watson equation. The age field is used in the male equation. Height and body weight are converted to the metric units required by the formula. The result is shown as a water volume and also as approximate water weight, using the practical approximation that 1 liter of water weighs about 1 kilogram.
Typical Total Body Water Percentages
Total body water is often described as a percentage of body weight. These are broad adult ranges. Your calculator result may differ because the Watson formula uses height, weight, age, and sex.
| Group | Typical TBW as % of body weight | Notes |
|---|---|---|
| Adult male | About 50% to 65% | Often higher because lean tissue contains more water. |
| Adult female | About 45% to 60% | Often lower on average due to differences in body composition. |
| Older adults | Often lower | TBW tends to decrease as lean mass decreases. |
Total Body Water Unit Conversions
| Conversion | Formula | Example using 40 L |
|---|---|---|
| Liters to cubic meters | L ÷ 1000 | 40 L = 0.040 m³ |
| Liters to cubic feet | L × 0.0353146667 | 40 L = 1.4126 ft³ |
| Liters to water weight | 1 L ≈ 1 kg | 40 L ≈ 40 kg, or 88.18 lb |
Example Calculations
Example 1: Male
Find the estimated total body water for a 40-year-old male who is 180 cm tall and weighs 80 kg.
The estimated total body water is about 44.86 liters. As water weight, this is about 44.86 kg or 98.91 lb.
Example 2: Female
Find the estimated total body water for a female who is 165 cm tall and weighs 60 kg.
The estimated total body water is about 30.33 liters. As water weight, this is about 30.33 kg or 66.87 lb.
FAQ
What does total body water mean?
Total body water is the estimated amount of water contained in your body. It includes water inside cells, water outside cells, blood plasma water, and other body fluids. It is not the amount of water you should drink in a day.
Is the Watson formula accurate for everyone?
No. The Watson formula is mainly validated for adults. It may be less accurate for children, teens, people with very high or very low body fat, pregnancy, major fluid imbalance, edema, dehydration, or serious illness. Treat the result as an estimate, not a diagnosis.
Why is water weight almost the same number as liters?
Water has a density close to 1 kilogram per liter. Because of that, 40 liters of water weighs about 40 kilograms. The calculator uses this practical approximation, then converts kilograms to pounds when you choose pounds.
