Free Water Deficit Calculator
Free Water Deficit Calculator - a deterministic tool in Sophie Well's Clinical Math & Conversions group.
Open the Free Water Deficit Calculator → Runs in your browser. No signup, no tracking.
What this is
Estimate the free-water deficit in hypernatremia using the classic total-body-water formula (0.6 x kg for adult men, 0.5 x kg for adult women and older men) and a target sodium, then convert that deficit into a replacement rate over a clinician-chosen window (default 48 hours). Sophie Well returns the deficit in litres plus the hourly rate for D5W or for an equivalent volume of enteral free water; it does not include ongoing insensible or urinary losses, which must be added on top.
When to use it
Use this at the start of a hypernatremia workup once volume status is known and you have committed to a correction window. The rule of thumb is to lower serum sodium by no more than 10-12 mEq/L per 24 h to avoid cerebral edema; this tile lets you pick a target Na (commonly 145) and a duration that respects that cap. It is for adult inpatient planning - neonatal and pediatric hypernatremia have different total-body-water fractions and different overcorrection risks; treat those with the dedicated peds resources, not this calculator.
References
Adrogue HJ, Madias NE. Hypernatremia. NEJM 2000;342:1493-1499.
Worked example: Free water deficit ~4.3 L; replace over 48 h.
Sophie Well is a reference and educational tool. Not medical, legal, or financial advice. Does not replace clinician judgment, professional billing review, or legal counsel.