Body Water Calculator
Calculate your total body water in litres or gallons with the Watson, Hume-Weyers and mL/kg methods. See your body water percentage, normal range, intracellular and plasma volumes, and implied lean mass.
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Fitness
Body Water Calculator
Calculate your total body water in litres or gallons with the Watson, Hume-Weyers and mL/kg methods. See your body water percentage, normal range, intracellular and plasma volumes, and implied lean mass.
Body Water Calculator
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Pick a method, choose your sex, and enter your weight (plus height and age unless you picked the mL/kg rule) to see your total body water, what share of your weight it is, where it sits against the normal range, and how the three equations compare.
Charts and reference values
Your body is mostly made up of water. This calculator shows how much of it is water in terms of volume (liters or gallons), weight of the water and percentage of your total body weight. Not only one formula is used, but three published formulas are calculated and all three results are shown at once so you can first check the differences between the results before carefully considering each value.
How much water is in your body?
The water content in healthy adult men is about 58%, with a range of ±8 percentage points. In healthy adult women it is around 48%, also with a range of ±6 percentage points. This difference is not gender specific, but depends on the fat content. The fat-free body consists of approximately three quarters water, while the fat portion only contains about one tenth water. At the same weight, women tend to have less water content relative to their body weight as they contain more essential fat than men.
This is why it is unreliable to estimate body water content based on the percentage of body weight alone. Even if two people weigh exactly the same, say 80 kg, they can differ by five or six liters in their total body water content depending on how much muscle mass is contained within those 80 kg.
Group | Typical body water | Why |
|---|---|---|
Newborn | About 75 percent | Very little fat, a lot of extracellular fluid |
Infant at one year | About 65 percent | Fat has been laid down; water share drops fast |
Adult man | 58 percent (50 to 66) | More muscle, less essential fat |
Adult woman | 48 percent (42 to 54) | More essential fat, which holds little water |
Man 65 and over | About 50 percent | Muscle loss with age |
Woman 65 and over | About 45 percent | Muscle loss with age |
The Watson Formula
Watson, Watson and Batt published these equations in 1980 that are used as standard in this calculator. They directly measured total body water by the isotope dilution method on 458 men and 265 women and derived these equations through regression analysis with respect to height, weight and age. These equations remain the most validated methods for estimating total body water from measurements of a tape measure and a scale.
For men, total body water is given in litres, height in centimetres and weight in kilograms.
For women...
Please pay close attention to the second formula. There is no age factor. Watson's regression analysis for women showed that once height and weight were included in the model, age was not a useful predictor, which is why the age parameter has been omitted from the published formula. When using the Watson method, the result for women does not change regardless of what age you enter. A calculation tool will show this point directly on the screen so there is no confusion.
For example, a man aged 30 years old, with a height of 180 cm and weight of 80 kg, if we do the calculation by subtracting 2,447 from 2,747, adding 19,332 then 26,896, it will give us an answer of 45.93 liters. This is equivalent to 57.4% of his weight which is within the normal range.
The Hume-Weyers Formula
Hume and Weyers published formulas using only height and weight in 1971, nearly a decade before Watson. These are still commonly used in clinical practice today, mainly for dialysis, to determine the target value for assessing Kt/V as it reflects total body water.
Since both formulas do not require an age input, Hume-Wheeler will give the same result for a 25 year old and a 75 year old. This is a simplification rather than a discovery; it's important to understand what inputs the formula actually uses.
If you use a man with the same height of 180 cm and weight of 80 kg, then the volume is 44.79 liters. For Watson it's 45.93 liters, so the difference is slightly over one liter, which corresponds to a relative difference of about 2.5%. This difference illustrates how accurate these results are.
A rule of thumb based on milliliters per kilogram
Nobody calculates a regression formula at the bedside. The rule of thumb is to multiply body weight by a fixed number of milliliters per kilogram. This value depends only on the patient's gender and age.
Group | Water per kilogram | Implied percentage |
|---|---|---|
Men under 65 | 600 mL/kg | 60 percent |
Women under 65 | 500 mL/kg | 50 percent |
Men 65 and over | 500 mL/kg | 50 percent |
Women 65 and over | 450 mL/kg | 45 percent |
Children | 600 mL/kg | 60 percent |
This is the only method that does not require height, so you can get a result even if you only have weight available. However, instead of showing an actual number it shows a percentage range which varies by gender and age group, so there's no way to tell whether someone is a bodybuilder or a sedentary person with the same weight. In this calculator tool we've set the upper limit for the senior factor at 65 years old.
Where is body water actually distributed?
The total body water balance is not a single amount of water but rather distributed in different areas, with this distribution being surprisingly stable from person to person.
About two thirds of it is inside cells and called intracellular fluid. This is the medium in which all chemical reactions in the body take place. The remaining one third is outside cells. Of this extracellular fluid, about four fifths is interstitial fluid surrounding tissues while one fifth is plasma.
A third volume of fluid, the transcellular fluid, includes cerebrospinal fluid, aqueous humor, synovial fluid and intestinal juice. Because it is such a small amount, it is completely ignored for calculations.
In a 45.93 litre example the intracellular fluid is about 30.6 litres, the interstitial fluid is 12.2 litres and plasma is 3.1 litres. This can also be used as an independent check since the actual measured volume of plasma in an 80kg man is about three litres.
Calculation of body water balance:
This is a factor that many body water calculators overlook. If the fat-free tissue holds a constant amount of water (73.2%), then knowing how much water you have will tell you your fat-free weight.
The rest is fat. This is not a surface technique but the underlying logic of the isotopic dilution method. Cheaper body fat measurements are validated using this method. Isotopic dilution is a reference method. The calculation tool displays these two values simultaneously so that you can convert an estimated amount of water in your body into an estimate of body composition for free.
Consider the estimated body fat percentage to be the least accurate value on this page. It inherits all of the errors that are contained in estimating water content, and amplifies them by dividing with 0.732. This is for plausibility checking purposes only and not a measure.
Factors that affect and do not affect body water balance:
Muscle affects body water content. When muscle increases, both the absolute volume and percentage increase since muscles are about 75% water. When fat increases, weight goes up but the percentage goes down.
The body's water balance changes slowly with age. After adulthood the amount of water in the body gradually decreases, mainly because of loss of muscle mass. This trend accelerates after 65 years of age as the response to thirst becomes lessened. Older people often do not feel thirsty until their bodies are already dehydrated.
Daily fluid intake has little effect on body water balance. Even in hot weather the fluctuation of total body water within a normal day is well under one percentage point. This is strictly regulated by the kidneys and thirst mechanism. Here is an important distinction: this calculator estimates water that was originally in your body, not forgotten cups of water. If you want to know your daily goals it's about calculating fluid intake which is a completely different issue.
How accurate are these results?
These are regression equations based on group data that show how much water people of similar size, weight, age and sex typically contain but they do not tell you how much water you actually contain.
The standard error of Watson's method itself is about two to three liters, and the differences between the three methods described on this page are usually also around that large. People with unusual body composition, such as very muscular people, extremely thin people, obese people, pregnant women, people with edema or dialysis patients are not included in the groups for which these equations were calibrated. So you should expect larger errors.
If an actual measurement is required rather than an estimate, there are isotopic dilution methods with heavy water or tritiated water and multi-frequency bioelectrical impedance analysis (BIA). Home body composition analyzers often use a less expensive single frequency version of BIA which can lead to fluctuations in the reported hydration status.
This calculator is for general educational purposes only and is not a substitute for medical advice. The estimation of total body water based on height, weight, age, and gender is an approximation only and should not be used in place of evaluation by a qualified physician. Do not use this as the basis for fluid intake, medication dosing or dialysis planning.
Frequently asked questions
- How much water is there in your body?
The water content of a healthy adult male is about 58% and for females it's around 48%. The normal range varies by ± 8 percentage points or ± 6 percentage points respectively. In newborns it's about 75%, while in adults over the age of 65 it falls to about 50% for males and 45% for females. When you enter your height, weight, age and gender into the fields above, the calculator will show your values in litres, kilograms and as a percentage.
- How is total body water calculated?
The best validated method is the Watson formula from 1980. The total body water (in liters) for men is calculated by adding 2.447 minus age times 0.09156, height in centimeters times 0.1074 and weight in kilograms times 0.3362. For women subtract 2.097, add height in centimeters times 0.1069 and weight in kilograms times 0.2466. This calculator also calculates the Hume-Wayers formula as well as the clinical practice rule of thumb of mL/kg so you can compare all three methods.
- Why doesn't the Watson formula for women have an age factor?
The original regression model found that there was no added value in keeping the age factor. As height and weight were included in the model, the contribution of age for the female sample was small so it has been omitted from the published formula. The male formula retains the age factor which means water content reduces by about 0.09 litres per year. This is a genuine limitation of the formula itself rather than an error on this site. The calculator tool shows this point on screen.
- What is the normal body water content?
In men it is about 50 to 66% while in women it is 42 to 54%. If this range is below, it usually does not indicate dehydration but a high body fat percentage. This is because the water content of fat is only about 10%. Even if this value is exceeded, it is normal for slim people, muscular people and young people. This is because the water content of muscles is about 75%. However in clinical situations a high value can also indicate fluid retention.
- Does drinking a lot of water increase your body's water content?
In most cases not much and even if it does rise, it doesn't stay up for long. Under normal circumstances the daily fluctuation of total body water is well below one percentage point. Excess water is normally excreted within a few hours because the kidneys and thirst mechanism are tightly regulated. Only muscle growth actually increases body water content. This is because muscles are about three-quarters water while fat is only about one-tenth water. Even if you reduce fat, the percentage goes up since the denominator is reduced.
- Is total body water content and amount of water I should drink the same?
No, they are different things and it is easy to confuse them. Total body water content is the amount of water that your body already contains, which is about 40-50 liters. Daily water intake is the amount you need to replace the fluids lost through urine, sweat and breathing, which is two to three liters. This calculator answers the first question while a fluid intake calculator would answer the second question.
- How much of the water in your body is plasma?
This is about one-fifteenth of the total body water. Two-thirds of the body's water is contained within cells while a third is outside the cells. Of this outer third, about one-fifth is plasma and four-fifths is interstitial fluid that penetrates tissues. In a typical 80 kg man, the plasma volume is about three liters, which agrees very well with directly measured values.
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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.
References
- Watson PE, Watson ID, Batt RD (1980): Total body water volumes for adult males and females
The Am J Clin Nutr paper behind the default equations, fitted on 458 men and 265 women.
- Hume R, Weyers E (1971): Relationship between total body water and surface area
The J Clin Pathol paper behind the height-and-weight alternative still used in dialysis.
- Brinkman JE, Dorius B, Sharma S: Physiology, Body Fluids (StatPearls)
NCBI Bookshelf chapter on body water as a share of body mass and the intracellular, interstitial and plasma compartments.
- Edelman IS, Leibman J (1959): Anatomy of body water and electrolytes
The Am J Med paper that established the compartment fractions and the body water percentages quoted here.
- Wang Z et al.: Hydration of fat-free body mass
The 0.732 hydration constant used to read fat-free mass back out of total body water.