Total Body Water Calculator
Enter age, sex, weight and height to estimate how much water your body holds. The equations that apply depend on age, and the guide explains which ones you see and why.
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Created by the Reeliy Editorial Team. Last updated: Oct 19, 2026
What total body water is
Total body water (TBW) is the volume of water in the whole body: blood, organs, muscle, bone and fat. Roughly two thirds sits inside cells and one third outside them, in blood plasma and the fluid between cells. Water makes up around half to three quarters of body weight depending on sex, age and body composition.
Clinicians use TBW to dose drugs that spread through body water, to judge fluid balance and, in kidney care, to work out how much dialysis a patient is receiving. It is not a hydration test: a person can be dehydrated or overhydrated at the same total volume. For a daily drinking target use the water intake calculator.
How to use the calculator
Choose a unit system, then enter age, sex, weight and height. The weight unit follows the unit system you select (pounds for US units, kilograms for metric). Results appear as litres with the equal weight in kilograms for metric units, or US fluid ounces with the weight in pounds for US units, and as a percentage of your body weight.
Age decides which equations are shown. Infants between one month and one year old should enter 1, because the child equations use height and weight rather than age.
| Equation | Shown for | Built for |
|---|---|---|
| Watson | Age 17 and over | Healthy adults |
| Mellits-Cheek | Under age 31 | Healthy children, teenagers and young adults |
| Morgenstern | Under age 31 | Children on peritoneal dialysis |
| Hume-Weyers | Every age | Adults of middle and older age |
Adults from 17 to 30 therefore see all four equations, while people of 31 and over see Watson and Hume-Weyers. A line giving the 95% range of body water percentage for your sex is added once Watson is shown.
The equations
All equations return litres. Age is in years, height (H) in centimetres and weight (W) in kilograms. One litre of water weighs one kilogram, so the same number is the weight of body water in kg; the calculator converts to fluid ounces (1 L = 33.814 US fl oz) and pounds (1 kg = 2.2046 lb) for US units.
Watson (adults)
Derived in 1980 from 458 men and 256 women aged 17 and over:
men: TBW = 2.447 − 0.09516 × age + 0.1074 × H + 0.3362 × W
women: TBW = −2.097 + 0.1069 × H + 0.2466 × W
The prediction error (standard deviation) is about 3.8 L for men and 3.6 L for women, so about 95 percent of estimates fall within roughly 7 L of a measured value.
Hume-Weyers
Based on 30 men and 30 women of middle to older age with no fluid retention or malnutrition:
men: TBW = 0.194786 × H + 0.296785 × W − 14.012934
women: TBW = 0.34454 × H + 0.183809 × W − 35.270121
Mellits-Cheek (children to young adults)
These equations switch to a second line above a cut-off height:
boys and men, H ≤ 132.7 cm: TBW = −1.927 + 0.465 × W + 0.045 × H
boys and men, H > 132.7 cm: TBW = −21.993 + 0.406 × W + 0.209 × H
girls and women, H ≤ 110.8 cm: TBW = 0.076 + 0.507 × W + 0.013 × H
girls and women, H > 110.8 cm: TBW = −10.313 + 0.252 × W + 0.154 × H
Morgenstern (children on dialysis)
A power-law fit from the Pediatric Peritoneal Dialysis Study Consortium, where the weight and height are multiplied before the exponent is applied:
boys: TBW = 0.074 × (H × W)0.66
girls: TBW = 0.117 × (H × W)0.59
Worked examples
A 45-year-old man, 180 cm, 85 kg. Watson: 2.447 − 0.09516 × 45 + 0.1074 × 180 + 0.3362 × 85 = 2.447 − 4.282 + 19.332 + 28.577 = 46.07 L, shown as 46.1 L. As a share of body weight that is 46.07 ÷ 85 = 54.2%. Hume-Weyers: 35.061 + 25.227 − 14.013 = 46.28 L, or 54.4%.
A 40-year-old woman, 165 cm, 65 kg. Watson: −2.097 + 17.639 + 16.029 = 31.57 L, which is 48.6% of her weight. Hume-Weyers gives 33.5 L, or 51.6%.
A 10-year-old boy, 140 cm, 35 kg. Mellits-Cheek: −21.993 + 0.406 × 35 + 0.209 × 140 = 21.5 L (61.4%). Morgenstern gives 20.2 L (57.6%). Hume-Weyers returns 23.6 L (67.6%), a reminder that it was not fitted to children and should not be relied on for them.
What body water percentage is normal?
Fat tissue holds much less water than lean tissue, so people with more body fat have a lower percentage. In the sample behind the Watson equation the average was 58.3% ± 6.7 for men and 48.5% ± 8.6 for women. Taking two standard deviations either side gives the ranges the calculator prints: 45.2% to 71.5% for men and 31.6% to 65.4% for women.
Percentages also change over a lifetime. These are rough rule-of-thumb figures from physiology teaching, not outputs of the calculator:
| Life stage | Typical body water share |
|---|---|
| Newborn | about 75% |
| Infant (6-12 months) | about 60-65% |
| Adult man | about 55-65% |
| Adult woman | about 45-55% |
| Older adult | a few points lower than in younger adults |
Limitations
- Obesity. Adipose tissue contains only 10 to 30 percent water, and equations fitted to people with a mix of body types tend to overestimate TBW in the very obese. A body fat estimate helps you judge whether that applies.
- Children and teenagers. Watson was not fitted to anyone under 17, which is why it is hidden for younger ages. Hume-Weyers was fitted to older adults only.
- Fluid shifts. Swelling, ascites, dehydration and kidney disease change the real value in ways that height and weight cannot capture.
- Population differences. The adult equations come from studies of mainly European-descent participants.
For electrolyte problems the related free water deficit calculator uses an estimate of TBW as one of its inputs.
Results are statistical estimates for general information and must not be used for medical decisions or treatment dosing. See our disclaimer.
Frequently asked questions
Why do I see four results when I am 25 but only two at 45?
The Mellits-Cheek and Morgenstern equations are shown only below age 31, while Watson needs age 17 or above. Hume-Weyers appears at every age.
What do I enter as age for a baby?
Enter 1 for any infant between one month and one year old. The child equations do not use age, and 1 simply makes them appear.
Is a higher body water percentage better?
Not by itself. The percentage mainly reflects how much of your weight is lean tissue versus fat. A lean athlete has a higher share than a person of the same weight with more body fat.
How many litres is that in fluid ounces?
Multiply litres by 33.814 to get US fluid ounces. For example, 46.1 L is about 1,559 fl oz.
References
- Watson PE, Watson ID, Batt RD (1980). Total body water volumes for adult males and females estimated from simple anthropometric measurements. American Journal of Clinical Nutrition 33(1):27-39.
- Hume R, Weyers E (1971). Relationship between total body water and surface area in normal and obese subjects. Journal of Clinical Pathology 24:234-238.
- Mellits ED, Cheek DB (1970). The assessment of body water and fatness from infancy to adulthood. Monographs of the Society for Research in Child Development 35(7):12-26.
Cite this calculator and page
If you use this page in your work, you can cite it as:
Reeliy Editorial Team. (2026). Total Body Water Calculator. Reeliy. https://reeliy.com/calculators/tbw-calculator“Total Body Water Calculator.” Reeliy, 2026, https://reeliy.com/calculators/tbw-calculator.