Total Body Water Calculator
Body water estimate via the Watson formula
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Total Body Water Calculator turns the numbers behind body water estimate via the Watson formula into a result, step by step. You supply Weight (kg), Height (cm) and Age (years) to Total Body Water; the answer follows at once. In Total Body Water the working sits beside the answer. Use Total Body Water for a reliable number, fast. No account is needed for Total Body Water. The browser does the arithmetic. Total Body Water sits in the Health section next to related tools and total body water calculator watson formula tbw hydration. The labels in Total Body Water match the terms in its result. Start with Total Body Water; the first number takes seconds.
Total body water (TBW) in litres
Total body water is the volume all of the body fluids sit in, and it scales with body size, age and sex. The figure sits behind hydration and drug-distribution calculations, so it is reported here both in litres and as a share of body weight.
That is roughly 59% of body mass, which is the normal range for a lean adult man.
The Watson formula
For men: TBW = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × weight (kg)
For women: TBW = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg)
For the defaults: 2.447 − 4.282 + 19.332 + 23.534 = 41.03 → 41.0 L.
Worked examples
70 kg, 180 cm, 45, male → 41.0 L
70 kg, 180 cm, 25, male → 42.9 L (age term is smaller)
60 kg, 165 cm, 35, female → −2.097 + 17.64 + 14.80 = 30.3 L
90 kg, 185 cm, 50, male → 49.5 L
Why anyone calculates this
Total body water is the volume some drugs distribute into, so a loading dose for a water-soluble drug can be scaled to it rather than to body weight. It is also used in dilution studies and to estimate body composition: fat holds little water, so a high TBW relative to weight suggests a leaner body, and a low one suggests more adipose tissue. Athletes routinely sit above the population average and older adults below it.
Limits of the estimate
- It is a population regression. Individuals vary by several litres at the same height and weight.
- Age is capped in practice. The equation was derived in adults; it is unreliable in children.
- Oedema and dehydration distort it. Fluid overload or significant loss breaks the assumption of normal hydration.
- Pregnancy and extreme body composition fall outside the derivation data.
Use the figure for understanding, not for dosing. Clinical fluid and drug calculations belong with a clinician.
The Watson formula is a population estimate, not a measurement. It puts a 70 kg, 180 cm man of 45 at about 41 litres, roughly 58% of body mass, and that share falls with age and rises with muscle. Clinical use is narrower: dosing for water-soluble drugs and estimating the volume in which a solute distributes. Hydration status, recent illness and body composition can move the true value several litres, so it is a planning figure rather than a diagnosis.
Frequently Asked Questions
What is a normal total body water for a 70 kg man?
About 40–43 litres, roughly 57–60% of body weight. The Watson formula gives 41.0 L for a 45-year-old at 180 cm.
Why does age reduce the estimate?
Because body water falls with age as muscle mass declines and fat proportion rises. The male equation subtracts about 0.1 L per year.
How accurate is the Watson formula?
It is accurate on average across a population but can be off by several litres for an individual, especially at extremes of body composition.
Is total body water the same as hydration status?
No. TBW is a fixed volume estimate; hydration is whether that volume is currently adequate, which needs a different measure.
Can I use this to size a water intake target?
Not directly. Intake depends on losses through urine, sweat and respiration, which vary far more than TBW does.
Why do women have a lower percentage?
The female equation gives a lower volume for the same weight because average body composition includes more adipose tissue, which holds little water.
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