About the Total Body Water Calculator
This total body water calculator estimates how much water your body contains, in litres and as a percentage of your body weight, using the Watson formula. It needs only your sex, age, height and weight, and also splits the total into the approximate intracellular (inside cells) and extracellular (blood plasma and fluid between cells) compartments.
Total body water (TBW) is used in nutrition, sports science and medicine — for example to understand hydration, to estimate how a substance spreads through the body, or as part of the Widmark blood-alcohol method. Water typically makes up about 50–60% of an adult’s weight; the share is higher in lean, muscular people because muscle holds far more water than fat.
The Watson equations were derived in 1980 from isotope-dilution measurements in healthy adults aged 18 and over. They are population estimates: they cannot detect dehydration or fluid retention, and they are less accurate in children, pregnancy, very high or low body fat, and conditions such as heart, liver or kidney failure. For a direct measurement, bioimpedance or dilution testing is needed.
With the default inputs, the total body water is 43.7 L. Change any value above to recalculate instantly.
How to use the total body water calculator
- 1Choose US or metric units.
- 2Select your sex and enter your age.
- 3Enter your height and weight.
- 4Read your total body water in litres and as a percentage of body weight.
- 5Compare the Watson estimate with the simple percentage rule of thumb.
Formula and method
Watson, Watson and Batt (1980) fitted these regression equations to 458 men and 265 women whose body water had been measured by isotope dilution. Height (H) is in centimetres, weight (W) in kilograms and the result in litres. Age improved the fit for men — body water falls as muscle mass declines with age — but added nothing for women, so the female equation has no age term.
Body water percentage divides litres by kilograms, treating 1 litre of body water as weighing 1 kg. The split of about two-thirds intracellular and one-third extracellular, with plasma making up about a quarter of extracellular fluid, is the standard physiological approximation.
- TBW
- Total body water in litres
- H
- Height in centimetres
- W
- Weight in kilograms
- age
- Age in years (men only)
Worked examples
40-year-old man, 5 ft 10 in, 170 lb
Height is 177.8 cm and weight 77.11 kg. TBW = 2.447 − 0.09516 × 40 + 0.1074 × 177.8 + 0.3362 × 77.11 ≈ 43.7 L, or about 56.6% of body weight — 29.1 L of it inside cells. The 60% rule of thumb would give 46.3 L.
30-year-old woman, 165 cm, 60 kg
TBW = −2.097 + 0.1069 × 165 + 0.2466 × 60 = −2.097 + 17.639 + 14.796 = 30.34 L — about 50.6% of her 60 kg, close to the 50% rule of thumb.
70-year-old man, 175 cm, 90 kg
The age term reduces the estimate: 0.09516 × 70 = 6.661. With 0.1074 × 175 = 18.795 and 0.3362 × 90 = 30.258, TBW = 2.447 − 6.661 + 18.795 + 30.258 = 44.84 L, about 49.8% of body weight — in line with the 50% rule of thumb for men over 65.
Frequently asked questions
What percentage of the body is water?+
In adults, water usually makes up about 50–65% of body weight for men and 45–60% for women. The share falls with age and rising body fat, because fat tissue holds far less water (roughly 10–30%) than muscle (about 75%).
What is the Watson formula?+
It is a pair of equations published by Watson, Watson and Batt in 1980 that predict total body water in litres from height, weight and (for men) age. It was calibrated against isotope-dilution measurements in healthy adults.
Can this tell me if I am dehydrated?+
No. The formula estimates how much water a typical body of your size holds, not how hydrated you are today. Thirst, urine colour, and in medical settings blood tests are better guides to hydration status.
Why do women have a lower body water percentage?+
On average women carry a higher proportion of body fat and less muscle than men of the same weight. Because fat holds little water, total body water as a share of weight is lower.
Where is the water stored?+
About two-thirds is inside cells (intracellular fluid). The remaining third is extracellular: roughly three-quarters of that sits between cells as interstitial fluid and about a quarter circulates as blood plasma.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.