How this instrument works
Watson, Watson, and Batt published this pair of equations in 1980, fitting them to total body water measured directly by dilution studies in 458 men and 265 women, then regressing that measured volume against height, weight, and age. For men: TBW = 2.447 − 0.09156 × age + 0.1074 × height + 0.3362 × weight. For women: TBW = −2.097 + 0.1069 × height + 0.2466 × weight, with height and weight in centimetres and kilograms and the result in litres. Because the equations use actual height and weight rather than one fixed ratio, two people who share a weight but differ in build get different estimates.
This site's free water deficit and sodium correction calculators use a simpler shortcut instead — a fixed fraction of body weight, roughly 0.6 for men and 0.5 for women, stepped down again past age 65. That fraction method is faster to compute by hand and remains widely used in acute settings for exactly that reason. Watson's full regression is generally considered the more individually tailored estimate of the two, since it responds to a person's actual height and weight rather than assuming everyone of a given sex and age bracket shares one ratio — though it needs the same handful of inputs either way, so the added detail costs nothing beyond entering two more numbers here.
One asymmetry in the original 1980 model deserves stating plainly rather than treated as an error: the men's equation includes an age term, so an older man's estimate runs a little lower than a younger man's at identical height and weight, reflecting typically reduced lean mass with age. The women's equation, exactly as Watson's team published it, carries no age term at all — a genuine feature of the fitted data, occasionally debated by later researchers, not an omission introduced by this calculator.
- Set Sex — it selects which of the two regression equations applies.
- Enter Age in years; this only affects the men's equation, which carries an age term.
- Enter Height in centimetres and Weight in kilograms.
- Read Total body water in litres; the working block shows each term of the sum.
Worked example — three bodies, three totals
A 30-year-old man, 175 cm and 75 kg: age term 0.09156 × 30 = 2.7468, subtracted from 2.447; height term 0.1074 × 175 = 18.795; weight term 0.3362 × 75 = 25.215. Sum: 2.447 − 2.7468 + 18.795 + 25.215 = 43.71 L.
A 30-year-old woman, 165 cm and 60 kg — no age term at all: height term 0.1069 × 165 = 17.6385; weight term 0.2466 × 60 = 14.796. Sum: −2.097 + 17.6385 + 14.796 = 30.34 L, noticeably less than the man above despite a similar age, driven by a smaller frame and the equation's own lower coefficients.
A 60-year-old man, 180 cm and 90 kg: age term 0.09156 × 60 = 5.4936; height term 0.1074 × 180 = 19.332; weight term 0.3362 × 90 = 30.258. Sum: 2.447 − 5.4936 + 19.332 + 30.258 = 46.54 L — a bigger frame outweighing the age penalty.
Questions
How does the Watson formula differ from the simpler fraction-of-weight method?
The simple method multiplies weight by one fixed ratio — about 0.6 for men and 0.5 for women, stepped down past 65 — the approach this site's free water deficit and sodium correction calculators use. Watson's formula instead runs a full regression across height, weight, and, for men, age, so it responds to an individual's actual build rather than applying the same ratio to everyone in a given sex and age bracket. The fraction method is faster and remains common in acute settings; Watson's version is generally regarded as the more tailored estimate.
Why does the men's equation include age but the women's doesn't?
Because that's how Watson, Watson, and Batt's 1980 regression came out when fitted separately to their male and female measurement data — the men's data showed a significant age relationship, with older men estimating slightly lower, while the women's data didn't show the same pattern strongly enough to earn its own term. It's a genuine, sometimes-debated feature of the original study's fitted model rather than an inconsistency introduced by this calculator.
What counts as a normal total body water percentage?
Water typically makes up somewhere around 50 to 65% of adult body weight, running higher in men and in leaner people, and lower in women and in those carrying more body fat, since fat tissue holds much less water than muscle does. Watson's formula reports a volume in litres rather than a percentage, but dividing the result by body weight converts it to one if that's what you need.
Can this formula be used for children?
No — it was derived from adult measurement data across the 458 men and 265 women in the original study, and body-water proportions in children follow a different pattern entirely, generally running higher than in adults. Pediatric total-body-water estimation uses separate, age-specific equations built for that purpose.
Does obesity affect how accurate this estimate is?
Yes, to some degree — like most anthropometric formulas built from a general adult population, Watson's equations were fit predominantly on body types typical of the era and cohort studied, and very high body-fat levels can shift the true water fraction meaningfully, because fat tissue itself holds relatively little water. The formula still returns a figure for any input, but treat it as a closer estimate near an average build and a rougher one at the extremes.
Why does total body water matter clinically?
It underlies dosing calculations for water-soluble medications, fluid-replacement planning in dehydration or hypernatremia, and dialysis prescribing, among other uses — several of which this site's own calculators build on. Knowing roughly how many litres of water a body holds turns a fluid or electrolyte correction from a rough guess into an estimate grounded in that person's actual size.
References
- Watson PE, Watson ID, Batt RD. 1980, Am J Clin Nutr — original Watson formula (PubMed)
- StatPearls — Physiology, Body Fluids (NCBI Bookshelf)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.