Total Body Water Calculator

What total body water is

You are, quite literally, mostly water. Total body water is the combined amount of it in you, inside your cells, in your blood, and in the fluid between everything. It adds up to more of your body weight than any other single thing, which is why it is a useful figure for understanding body composition.

This calculator estimates that amount from your age, sex, height and weight, and reports it two ways: as a volume in litres, and as a percentage of your body weight. Below is what a typical figure looks like, the formula behind it, and one important thing this number is not.

How much water is in the body?

For a healthy adult, total body water usually lands somewhere around 50 to 65 percent of body weight. But it varies in ways that are worth understanding, because they explain your result:

• Sex. Men average higher, around 60 percent, and women a little lower, often nearer 50 to 55 percent. The reason is body composition: fat tissue holds much less water than muscle, and women carry proportionally more fat.
• Body composition. The same logic applies to anyone. More muscle means more water, more fat means proportionally less.
• Age. Babies are the wateriest of all, around 75 percent, and the fraction gently declines with age as body composition changes.

So a lower percentage is not automatically a bad sign. It often just reflects carrying more body fat, which is why this figure is best read alongside other composition numbers rather than on its own.

The formula this calculator uses

The result comes from the Watson formula, published by Watson and colleagues in 1980, which remains one of the most widely used ways to estimate body water from simple measurements. It uses separate equations for each sex, with height in centimetres and weight in kilograms:

Men: TBW = 2.447 − (0.09156 × age) + (0.1074 × height) + (0.3362 × weight)
Women: TBW = −2.097 + (0.1069 × height) + (0.2466 × weight)

You may notice the women's equation does not use age at all. That is not an oversight on our part; it is how Watson's original formula was built, because in their data age did not improve the estimate for women the way it did for men. The tool then divides the result by your weight to give the percentage.

A worked example

Take a 30 year old man, 178 cm tall, 75 kg. Into the men's equation:

TBW = 2.447 − (0.09156 × 30) + (0.1074 × 178) + (0.3362 × 75)
TBW = 2.447 − 2.75 + 19.12 + 25.22
TBW ≈ 44.03 litres

As a share of his 75 kg, that is about 59 percent, right in line with the typical male figure. Because a litre of water weighs almost exactly a kilogram, the tool also shows that volume as roughly 44 kg of your body weight.

What this does not tell you

This is the point worth pausing on. It is easy to assume a "body water" number tells you whether you are hydrated right now. It does not.

What the Watson formula estimates is a fairly stable, structural quantity: roughly how much water your body holds given your size and composition. It is not a live reading of your hydration state, and it will not tell you to drink a glass of water. Whether you are actually dehydrated on a given afternoon depends on what you have drunk, sweated and eaten, and shows up in signs like thirst, urine colour and how you feel, none of which this calculation can see. If you want to think about daily fluid needs instead, that is a different question, and the Water Intake Calculator is the tool for it.

Where the estimate can be off

Like any formula built on population averages, it has honest limits:

• It cannot see your body composition directly. Two people of the same age, sex, height and weight get the same figure, even if one is far more muscular, and muscle holds more water than fat.
• It is built for healthy adults. Conditions that change fluid balance, such as heart, kidney or liver problems, pregnancy, or the medications used to treat them, can move real body water well away from the estimate.
• It is a snapshot of structure, not status. As above, it estimates how much water your body typically holds, not whether you are currently over or under hydrated.

As a general composition estimate it is genuinely useful. If body water matters for a medical reason, such as a kidney condition or a dosing question, that belongs with a doctor who can measure and interpret it properly.

Questions people ask

What is a normal body water percentage?

Broadly, about 50 to 65 percent of body weight for adults, with men usually toward the higher end and women a little lower. There is no single target, and a figure outside that band is not automatically a problem, since it largely tracks your body composition.

Does drinking more water raise my percentage?

Only very briefly. Your body tightly regulates its water, so drinking extra mostly ends up passed as urine rather than stored. This calculation reflects your longer-term structure, not the glass you just drank, so it will not climb because you hydrated this morning.

Why does the women's version ignore age?

Because the original Watson formula was built that way. When the researchers fit their data, adding age improved the estimate for men but not meaningfully for women, so the female equation leaves it out. It is a feature of the formula, not a bug in the tool.

References

Where the figures come from. The equations this tool runs are the Watson formulas, from Watson and colleagues' 1980 paper, reproduced above as the tool applies them. The general context on body water as a share of body weight and on hydration draws on the Institute of Medicine's dietary reference report on water.

1. 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(1):27-39. https://doi.org/10.1093/ajcn/33.1.27
2. Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academies Press; 2005.