Body Water Calculator
Estimate total body water using the clinical Watson (1980) equation, with a Hume–Weyers cross-check and the standard intracellular/extracellular compartment split.
Total body water (Watson)
43.7 L
- Body water as % of weight
- 58.3%
- In US gallons
- 11.55 gal
- Intracellular fluid (≈60%)
- 26.2 L
- Extracellular fluid (≈40%)
- 17.5 L
- Interstitial fluid (≈75% of ECF)
- 13.1 L
- Plasma volume (≈25% of ECF)
- 4.4 L
- Hume–Weyers comparison
- 42.3 L
Watson (1980) estimates about 43.7 L of total body water, around 58% of body weight. About 26.2 L is held inside cells and 17.5 L sits in the extracellular space (interstitial fluid + plasma). The Hume–Weyers formula gives 42.3 L as a cross-check — the two usually agree to within a litre or so for typical adults.
How to use this calculator
Pick male or female (the Watson coefficients differ — male equation includes age, female equation does not), then enter age in years, height in centimetres and weight in kilograms. Defaults reflect a representative adult male at 30 years, 175 cm and 75 kg. The headline number is total body water in litres using the Watson formula. The breakdown shows body water as a percentage of body weight (typically 50–65% in adults), the same volume in US gallons, the intracellular/extracellular split (about 60/40 of TBW), interstitial fluid and plasma volumes within the extracellular space (75/25 of ECF), and the Hume–Weyers estimate as a sanity check. The two formulas usually agree to within a litre for typical adults; bigger gaps tend to flag inputs at the edges of the validation cohort (very obese, very elderly, very short).
How the calculation works
Total body water (TBW) is the sum of every water molecule in the body — about two-thirds tucked inside cells (intracellular fluid, ICF) and one-third bathing the cells (extracellular fluid, ECF, which is itself mostly interstitial fluid with about a quarter as blood plasma). Direct measurement uses deuterium- or tritium-dilution and is impractical outside a research lab, so clinicians estimate TBW from anthropometric data. The Watson formula (Watson, Watson & Batt, Am J Clin Nutr 1980) is the most widely cited estimator: it was fit on 458 healthy adults and gives separate equations for men (TBW = 2.447 − 0.09156·age + 0.1074·height_cm + 0.3362·weight_kg) and women (TBW = −2.097 + 0.1069·height_cm + 0.2466·weight_kg). Notice the male equation includes age — TBW declines about 0.09 L per year as lean mass falls — while the female equation does not, because the original cohort showed no significant age effect once height and weight were accounted for. Hume–Weyers (J Clin Pathol 1971) is a slightly older two-variable equation we report as a comparison value. The 60/40 ICF/ECF split is from Guyton & Hall’s Textbook of Medical Physiology and is the conventional teaching value for healthy adults.
Worked example
A 30-year-old man who is 170 cm tall and weighs 70 kg: TBW = 2.447 − 0.09156 × 30 + 0.1074 × 170 + 0.3362 × 70 = 2.447 − 2.747 + 18.258 + 23.534 = 41.5 L. That is about 59% of body weight, which is right in the middle of the normal 55–65% adult-male range. Of those 41.5 L, roughly 24.9 L is intracellular and 16.6 L extracellular; within the ECF, about 12.5 L is interstitial fluid and 4.2 L is blood plasma — close to the textbook 5 L circulating blood volume once you add the red-cell volume. For a 30-year-old woman, 165 cm, 60 kg: TBW = −2.097 + 0.1069 × 165 + 0.2466 × 60 = 30.3 L, or 50.6% of body weight — typical for adult females, whose higher body-fat fraction means lower water as a share of mass.
Frequently asked questions
What is total body water and why estimate it?
Total body water (TBW) is the total volume of water in the body — roughly 50–65% of body weight in healthy adults. It is split between intracellular fluid inside cells (about 60% of TBW) and extracellular fluid outside cells (40%, made up of interstitial fluid and blood plasma). Knowing TBW matters in clinical settings: it is the V in the Kt/V dialysis-adequacy formula used to prescribe haemodialysis, it sets the volume of distribution for water-soluble drugs (so doses scale with TBW, not body weight), and it is part of how anaesthetists and intensivists estimate fluid replacement after burns and major surgery.
Which formula does this calculator use?
The primary number is the Watson formula (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(1):27–39). Watson is the most widely cited TBW estimator and is the one recommended by the NKF KDOQI dialysis guidelines for computing single-pool Kt/V. As a sanity check we also report the Hume–Weyers formula (J Clin Pathol 1971;24(3):234–238), which uses only height and weight. The two usually agree to within a litre for typical adult body sizes.
Why does the male Watson equation include age but the female one does not?
In the original Watson cohort, total body water declined measurably with age in men (about 0.09 L per year), reflecting the gradual loss of lean tissue with ageing — lean tissue is roughly 73% water, fat tissue much less. In women, height and weight already absorbed most of that variance once you accounted for the higher average body-fat fraction, and the residual age coefficient was not statistically significant. Watson kept the equations parsimonious: only include a term if the data demand it. So the male equation has four terms, the female equation three.
What is a normal body-water percentage?
For adult men, total body water is typically 55–65% of body weight; for adult women, about 50–60%. Women run lower because they carry a higher proportion of body fat, and fat tissue is only about 10–20% water. Body water as a percentage of weight tends to fall with age in both sexes — newborns are about 75% water, healthy young adults about 60%, the elderly closer to 50%. Athletes and very lean individuals can sit at the top of the range; obese individuals at the bottom, because their excess weight is mostly low-water adipose tissue.
When is this calculator NOT appropriate?
The Watson and Hume–Weyers formulas were validated on healthy adults aged roughly 18 to 70, with body sizes inside a normal-to-overweight range. They become less reliable for children (whose body composition is very different — use the Mellits–Cheek paediatric formula instead), very obese patients (where bioimpedance analysis or deuterium dilution is more accurate), pregnant women (TBW rises by about 7–8 L by term, well outside the model), patients with significant fluid overload or dehydration (ascites, oedema, severe diuresis), and amputees or patients with major limb loss (height-based equations over-estimate TBW). Treat the result as an estimate, not a clinical measurement.
What is the difference between body water and daily water intake?
Body water (this calculator) is the total static volume of water already in your body — about 40 L for a typical adult. Daily water intake is the dynamic flux: how much water you need to drink each day to replace losses from urine, sweat, breath and stool — typically 2 to 3 L of fluid from all sources for a healthy adult. The two are related (intake replaces TBW turnover) but completely different quantities. For daily intake targets, use the Calc Dragon water intake calculator instead.