Free Water Deficit Calculator
Free water deficit for hypernatremia: TBW*(currentNa/targetNa - 1), replacement rate with 10 mEq/L/24h safety cap.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Weight
- 70 kg
- Sex
- Male
- Current serum Na
- 160 mEq/L
- Target Na
- 145 mEq/L
- Replace over
- 48 hours
Result: Free water deficit ~4.3 L; replace over 48 h.
The tool opens with these values already filled in. Replace them with your own.
What you enter
- Weight
- Sex
- Current serum Na
- Target Na
- Replace over
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 tool 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.
How this is calculated
Adrogue HJ, Madias NE. Hypernatremia. NEJM 2000;342:1493-1499. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.
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