Roughly half your body is water, but roughly is not good enough for dialysis dosing or sodium correction. The Watson equations estimate your total water volume from sex, age, height, and weight.
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Scientific references
The ocean you carry
An average adult man carries about 42 liters of water, an average woman near 30, spread across cells, blood, and the fluid bathing every tissue. The share of body weight that is water falls with age as muscle gives way to fat, and it runs lower in women at every age because of typically higher fat fractions. These patterns are consistent enough that anthropometric equations can estimate the volume within a few liters for most people.
The Watson equations
In 1980 Watson and colleagues measured total body water by isotope dilution in hundreds of healthy adults and regressed the results against simple measurements. The male equation credits height and weight while subtracting for age, capturing the drying that comes with the decades. The female equation leans on height and weight alone. Both remain standard references forty years on, embedded in dialysis machines and clinical calculators everywhere.
Where the number works for a living
Dialysis adequacy is judged by Kt/V, where V is exactly this water volume, so every hemodialysis prescription in the world touches a Watson like estimate. Sodium disorders get corrected with formulas that ask how much water the sodium is dissolved in, and the answer comes from here. Research on hydration status and body composition anchors on it too.
Beyond the estimate
Bioimpedance scales measure water directly by passing a tiny current through the body, and they agree with Watson reasonably in healthy people. The agreement collapses in edema, ascites, pregnancy, and severe obesity, situations where the assumptions behind the regression no longer hold. For everyday curiosity and standard clinical math though, this estimate remains the dependable workhorse it has been since 1980.