Free Water Deficit Calculator

Free water deficit estimates how many liters of solute-free water a hypernatremic patient has lost, on the assumption that total body sodium content has not changed. Because sodium is the dominant extracellular cation and water moves freely between compartments, serum sodium behaves as an inverse marker of body water. The product of total body water and sodium concentration is treated as conserved, so current TBW times current sodium equals normal TBW times target sodium; rearranging gives Deficit = TBW x (Current Na / Target Na - 1). Total body water is not measured but estimated from weight, using 0.60 for adult men, 0.50 for adult women and elderly men, and 0.45 for elderly women, reflecting the fact that lean tissue is about 73 percent water while adipose tissue is roughly 10 percent. Hypernatremia begins above 145 mEq/L and becomes clinically urgent above about 160 mEq/L. The rate limit matters more than the total volume: chronic hypernatremia should be corrected by no more than about 10 mEq/L in 24 hours, because brain cells accumulate idiogenic osmoles over 24 to 48 hours and lowering serum sodium faster than those osmoles can clear draws water into neurons and causes cerebral edema and seizures. The formula's largest practical weakness is that it ignores ongoing losses. Insensible losses of 800 to 1000 mL daily, and urine output that can exceed 10 L per day in diabetes insipidus, must be added on top of the calculated deficit, and serum sodium should be rechecked every 4 to 6 hours during active correction.

The formula rests on a quantitative picture of body fluid compartments assembled in the 1940s and 1950s, when isotope dilution first made total body water measurable. Deuterium oxide and antipyrine dilution studies, together with Francis Moore's body composition work at the Peter Bent Brigham Hospital in Boston, established that water makes up roughly 60 percent of body mass in young men and less in women and older adults, with the difference tracking adipose tissue rather than sex as such. In 1958 Isidore Edelman and colleagues published the relationship between serum sodium concentration and total exchangeable sodium, potassium, and body water, which is the empirical foundation for treating serum sodium as a readout of body water rather than of body sodium. Clinical use of a calculated water deficit spread through nephrology and intensive care in the following decades, alongside growing recognition that the brain adapts to chronic hypertonicity. Animal and clinical work in the 1970s and 1980s identified the accumulation of organic osmolytes in brain tissue during sustained hypernatremia and the risk of edema on rapid correction, which is why modern practice pairs the deficit calculation with an explicit rate ceiling rather than treating it as a volume to be infused quickly. Horacio Adrogue and Nicolaos Madias consolidated this reasoning into practical bedside guidance in their 2000 New England Journal of Medicine review of hypernatremia, which remains the framework taught today.