Water is the most abundant substance in the human body. It fills the cells, bathes the tissues and makes up most of the blood. How much of it you carry depends on how much of your weight is lean tissue, because muscle holds a lot of water and fat holds very little. Total body water is therefore a clue to body composition, and doctors use it in a few specific situations, such as setting the dose of dialysis and interpreting fluid balance. This calculator estimates it from your sex, age, height and weight with two well-known prediction formulas.
These are population-based estimates. They give a plausible volume for a healthy adult of your build, not a measurement of your hydration on any given day.
How to use the body water calculator
- Select your sex and enter your age.
- Enter your height and weight in any units.
- Read the Watson and Hume-Weyers estimates, in litres or another volume unit, and the average.
- Check the percentage of body weight, which shows whether you are near the typical range.
Total body water formulas
Age is in years, height H in centimetres, weight W in kilograms; results are in litres.
Watson, women: TBW = −2.097 + 0.1069 × H + 0.2466 × W
Hume-Weyers, men: TBW = 0.194786 × H + 0.296785 × W − 14.012934
Hume-Weyers, women: TBW = 0.34454 × H + 0.183809 × W − 35.270121
Watson PE, Watson ID, Batt RD, Total body water volumes for adult males and females estimated from simple anthropometric measurements, American Journal of Clinical Nutrition 1980;33:27–39. Hume R, Weyers E, Relationship between total body water and surface area in normal and obese subjects, Journal of Clinical Pathology 1971;24:234–238.
Worked example
A 35-year-old man, 180 cm and 80 kg.
- Watson = 2.447 − 0.09156 × 35 + 0.1074 × 180 + 0.3362 × 80 = 2.447 − 3.205 + 19.332 + 26.896 = 45.5 L
- Hume-Weyers = 0.194786 × 180 + 0.296785 × 80 − 14.013 = 44.8 L
- As a share of body weight, the Watson value is 57%.
A 30-year-old woman, 165 cm and 62 kg: Watson 30.8 L, Hume-Weyers 33.0 L, which is 50% of her weight.
Total body water by age and sex
For an adult man of 175 cm and 75 kg and an adult woman of 165 cm and 62 kg.
| Sex | Age | Watson | Hume-Weyers | Watson as % of weight |
|---|---|---|---|---|
| Man | 25 | 44.2 L | 42.3 L | 59% |
| Man | 45 | 42.3 L | 42.3 L | 56% |
| Man | 65 | 40.5 L | 42.3 L | 54% |
| Woman | 25 | 30.8 L | 33.0 L | 50% |
| Woman | 45 | 30.8 L | 33.0 L | 50% |
| Woman | 65 | 30.8 L | 33.0 L | 50% |
What affects body water
- Body composition. More fat means a lower percentage; more muscle a higher one.
- Age. Water falls as a share of weight with age as muscle declines, which is why the Watson men’s equation includes an age term.
- Sex. Women usually have a lower share of water because of a higher average body fat percentage.
- Fluid status. Dehydration, oedema, heart or kidney failure and some medicines change body water. The formulas assume a healthy fluid balance.
Cautions and related tools
The equations were built from measurements in healthy adults using tracer dilution and are less accurate in obesity, in older age and in illness. They estimate the amount of water, not whether you drink enough. For daily intake targets use the Water Intake Calculator. For related measures of composition, see the Body Fat Calculator, Lean Body Mass Calculator and BMI Calculator, and the Body Surface Area Calculator for another height-and-weight measure.
This page is for information only and is not medical advice. Speak to a doctor about your own health and any treatment decisions.
Sources
- Watson PE, Watson ID, Batt RD. Total body water volumes for adult males and females estimated from simple anthropometric measurements. Am J Clin Nutr 1980;33:27–39.
- Hume R, Weyers E. Relationship between total body water and surface area in normal and obese subjects. J Clin Pathol 1971;24:234–238.
Frequently asked questions
What is total body water?
Total body water (TBW) is the amount of water in the whole body, in the cells and in the fluid around them. In adults it makes up roughly half to two thirds of body weight, depending on age, sex and how much fat the person carries, because fat tissue holds little water.
Why do men have more body water than women?
On average men have more muscle and less body fat than women of the same weight. Muscle is about 75 percent water and fat only about 10 to 20 percent, so a more muscular body holds proportionally more water.
Which formula is more accurate?
Both were derived by comparing measurements with dilution methods in healthy adults. Watson is the most widely used in clinical research, particularly in dialysis, while Hume-Weyers is an older alternative. They typically differ by one or two litres, and both are less reliable for people with obesity, oedema or very low weight.
Does drinking more water raise total body water?
Only temporarily. A healthy body regulates its water tightly, and extra fluid is excreted by the kidneys. Total body water changes with body composition, age, illness and dehydration.
Can this be used to judge hydration?
No. These formulas estimate the typical water volume for a person of your size and do not measure how hydrated you are today. See the Water Intake Calculator for drinking guidance, and consult a clinician for medical concerns.