A total body water calculator estimates how many litres of water a body contains, using regression equations built from measurements of real people. It is not a measurement. Actual total body water is determined by isotope dilution — a person drinks a labelled dose of water and the concentration is measured after it distributes — and no equation based on height, weight, age and sex can reproduce that for an individual. What the equations do is predict a population average for people with those characteristics.
This page runs two of them side by side, because a single number implies a precision that is not there. The Watson equations from 1980 and the Hume-Weyers equations from 1971 were derived on different samples and give different answers for the same person, sometimes by a litre or two. Arb Digital publishes free calculators that show their own uncertainty rather than hiding it, and on this page the disagreement between the two methods is presented as a result in its own right.
What This Total Body Water Calculator Does
Enter age, sex, height and weight, and the calculator returns total body water in litres from the Watson equation as the headline figure, that volume as a percentage of body mass, the Hume-Weyers estimate for comparison, the difference between the two, and the mass of that water in kilograms or pounds. The bars show water against the rest of body mass so the proportion is visible.
Two other tools on this site sit close to this one and do different jobs, so it is worth being exact. The lean body mass calculator estimates lean mass — everything that is not fat, in kilograms — using the Boer formula. This page estimates body water in litres, which is a different quantity: water sits mostly inside lean tissue but lean mass also includes protein, mineral and glycogen. The water weight calculator uses the simpler percentage-of-body-weight approach and is written around day-to-day fluctuation on the scale. This page implements the published anthropometric equations and reports their disagreement.
How to Use It
- Choose your units. Both equations were published in metric. Imperial entries are converted to kilograms and centimetres before the equation runs, so the result is identical either way.
- Select the equation set. Watson and Hume-Weyers each published separate male and female coefficients derived from separate samples. The selector chooses which published set is applied; there is no third published set.
- Enter age. Only the Watson male equation uses it. The Watson female equation has no age term at all, and neither Hume-Weyers equation does, so changing age will move one of the four numbers and not the others.
- Enter height and weight. Total body weight, not lean mass — the equations were regressed against total mass.
- Read the gap. If the two estimates differ by a litre, that gap is a better description of the uncertainty than either figure on its own.
The Formula — How It's Calculated
The Watson equations, published by Watson, Watson and Batt in the American Journal of Clinical Nutrition in 1980, are stated as follows, with height in centimetres, weight in kilograms, age in years and the result in litres:
Men: TBW = 2.447 − (0.09516 × age) + (0.1074 × height) + (0.3362 × weight).
Women: TBW = −2.097 + (0.1069 × height) + (0.2466 × weight).
The Hume-Weyers equations, published in 1971, use the same units and no age term:
Men: TBW = (0.194786 × height) + (0.296785 × weight) − 14.012934.
Women: TBW = (0.34454 × height) + (0.183809 × weight) − 35.270121.
Water as a share of mass is the litre figure divided by weight in kilograms, using the standard approximation that one litre of water has a mass of one kilogram. That approximation is close enough at body temperature for this purpose; water is slightly less dense when warm than at its 4°C maximum.
Worked example. A 40-year-old man, 178 cm, 80 kg. Watson gives 2.447 − (0.09516 × 40) + (0.1074 × 178) + (0.3362 × 80) = 2.447 − 3.806 + 19.117 + 26.896 = 44.65 litres. As a share of mass that is 44.65 ÷ 80 = 55.8%. Hume-Weyers gives (0.194786 × 178) + (0.296785 × 80) − 14.012934 = 34.672 + 23.743 − 14.013 = 44.40 litres. The two differ by 0.25 litres here, which is unusually close — at other heights and weights the gap widens considerably.
How Wrong These Equations Can Be
This is the part most calculators leave out. The Watson equation was derived by regression against dilution measurements, and it carries a standard error that is not small relative to the quantity. More importantly, its accuracy varies systematically with body composition rather than randomly.
A published comparison of the Watson equation against bioimpedance spectroscopy in chronic haemodialysis patients found a strong correlation between the two methods but a consistent bias: the Watson formula overestimated total body water by roughly 4.6 litres on average — about 5.1 litres in men and 3.8 litres in women — with the overestimation most pronounced in patients with higher abdominal obesity and fat mass. That study population was not the general public, and the size of the bias in a healthy population would differ. But the direction of the problem is general and predictable.
The reason is structural. Fat tissue holds far less water than lean tissue does, but the equation only sees total weight — it cannot tell whether an extra ten kilograms is muscle or fat, and it applies the same coefficient either way. So the equation tends to overestimate in people carrying more fat than the derivation sample and underestimate in leaner people. Anyone whose body composition is unusual for their height and weight is exactly the person for whom the estimate is least reliable, which is an awkward property for a predictive equation to have.
Why Men and Women Get Different Equations
Not because of anything mysterious, but because average body composition differs and water lives in lean tissue. The StatPearls chapter on water balance gives the standard figure that for an average 70 kg man, about 60% of body weight is water, amounting to roughly 42 litres. Female averages sit lower, principally because average body fat percentage is higher, and fat is a poor water reservoir.
That is a population average being applied to an individual, with all the caveats that implies. A very lean woman can have a higher water percentage than an average man; a man carrying substantial fat can sit well below the 60% figure. The equations encode averages of the samples they were built on, which is why the selector chooses between published coefficient sets rather than describing anyone.
Age appears in the Watson male equation with a negative coefficient — roughly 0.095 litres per year — reflecting the observed decline in water as a share of mass across adulthood, largely because the ratio of lean to fat tissue tends to shift with age. The Watson female equation has no age term at all. That asymmetry is in the published equations, not something this page introduced, and it is one reason the two sexes' estimates are not directly comparable in structure.
Intracellular and Extracellular: What the Single Number Hides
Total body water is a sum of two compartments that behave very differently. Roughly two thirds sits inside cells as intracellular fluid; the remaining third is extracellular, split between the fluid between cells and the plasma in the bloodstream. These proportions are themselves population approximations.
The distinction matters because the compartments are what change independently in real situations. A single total-litres figure cannot distinguish between them, and neither Watson nor Hume-Weyers attempts to — both predict the total only. Bioimpedance devices attempt the split by measuring at multiple frequencies, and multi-frequency and single-frequency devices frequently disagree with each other. Any tool reporting a single number, including this one, is reporting a sum whose composition it does not know.
What This Number Is Not For
It is not a hydration reading. Total body water is a structural quantity — how much water a body of this size and composition contains — and it says nothing about whether a person is currently well hydrated. A hydration state is a short-term deviation from an individual's own baseline, and this equation does not know that baseline, cannot measure a deviation from it, and would return the same answer whether the deviation existed or not.
Nor does this page set any intake figure, and it deliberately does not compute one. Fluid needs are affected by activity, climate, diet, medication, kidney and heart function and much else, none of which appear in these equations. The water intake calculator is a separate tool covering the intake question on its own terms; it is a different calculation with different inputs and nothing on this page feeds into it.
For related body-composition estimates, the body fat calculator and the BMI calculator use different published methods, and the BMR calculator covers resting energy expenditure. All of them share the same limitation as this page: they are equations fitted to samples, not measurements of you.
Arb Digital's free tools library covers hundreds of everyday calculations, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Reading it as a hydration measurement — it estimates how much water a body of this size contains, not whether that body is currently short of water.
- Entering lean mass instead of body weight — the equations were regressed against total body mass and will return nonsense from a lean-mass figure.
- Quoting one equation as the answer — Watson and Hume-Weyers disagree, and the gap between them is part of the result.
- Applying it at the extremes — the equations are least reliable for people whose body composition is unusual for their height and weight, in a predictable direction.
- Assuming a litre is exactly a kilogram — it is an approximation that is close enough here but is not an identity.
Related Free Tools From Arb Digital
Use the lean body mass calculator for lean mass in kilograms rather than water in litres, the water weight calculator for the simpler percentage approach and day-to-day scale movement, the body fat calculator for the fat side of the same composition question, the BMI calculator for the height-and-weight index, and the water intake calculator if intake rather than volume is what you came for. Everything else is in the free online tools hub.
Frequently Asked Questions
For men it is 2.447 minus 0.09516 times age, plus 0.1074 times height in centimetres, plus 0.3362 times weight in kilograms. For women it is minus 2.097, plus 0.1069 times height, plus 0.2466 times weight. Both were published by Watson, Watson and Batt in the American Journal of Clinical Nutrition in 1980 and return litres.
It is a regression prediction, not a measurement, and the error is systematic rather than random. One published comparison in haemodialysis patients found the Watson equation overestimated total body water by around 4.6 litres on average, with the largest overestimates in people carrying more abdominal fat.
No. It estimates how much water a body of this size and composition holds in total. Hydration state is a short-term deviation from an individual baseline that this equation does not know and cannot detect, and the answer would be the same either way.
Because only the Watson male equation contains an age term. The Watson female equation and both Hume-Weyers equations have no age coefficient at all, so changing age moves one of the four published estimates and leaves the others where they were.
Lean body mass is everything that is not fat, reported in kilograms, and this site's lean body mass calculator uses the Boer formula for it. This page estimates water volume in litres. Water sits mostly within lean tissue, but lean mass also includes protein, mineral and glycogen, so the two are different quantities.
They were derived by regression on different samples of people at different times, so their coefficients differ. Neither is the correct one. The gap between them is a reasonable indication of how much confidence any single anthropometric estimate deserves.
The commonly cited figure is about 60% for an average 70 kg man, which works out to roughly 42 litres. That is a population average and individuals vary considerably, mainly because fat tissue holds much less water than lean tissue does.
No, and the page deliberately does not compute an intake figure. Fluid requirements depend on activity, climate, diet, medication and health conditions, none of which appear in these equations.
This tool applies published anthropometric equations to figures you enter, for general information only. It is not medical advice, it does not measure hydration, and it does not indicate any condition. Any question about body water, fluid balance or how much to drink — particularly for anyone with a kidney, heart or liver condition — should go to a qualified doctor.
