Free Water Deficit Calculator
Use this free water deficit calculator to estimate the free water deficit in hypernatremia using body weight, current serum sodium, target sodium, estimated total body water and planned correction time.
This tool is for clinical education. It does not replace bedside assessment, volume status, urine output, ongoing losses, kidney function, glucose correction, sodium trend, local protocol or clinician judgment.
Quick answer: what is free water deficit?
Free water deficit is the estimated amount of electrolyte-free water needed to lower an elevated serum sodium toward a chosen target sodium. It is commonly used in hypernatremia, where serum sodium is high because body water is low relative to total body sodium.
The calculator estimates the existing deficit only. It does not automatically solve ongoing free water losses, urine losses, diarrhea, fever, osmotic diuresis, insensible losses, shock, volume depletion or the safest fluid choice.
Free Water Deficit Calculator
Enter weight, current sodium and target sodium. The calculator uses estimated total body water and shows a conservative correction warning.
Ongoing losses field is optional and adds extra liters to the replacement planning table. It does not replace real-time urine output, stool loss, fever and fluid balance tracking.
Correction attention score
Review: Check correction rate, duration, symptoms, volume status and ongoing losses.
Replacement planning table
This table divides the estimated deficit plus optional ongoing losses across common time windows. It is not an order set. Fluid selection and rate require clinical judgment.
| Replacement window | Total volume | Approximate hourly rate | Use note |
|---|---|---|---|
| 24 hours | 4.3 L | 179 mL/hr | May be too fast for chronic or unknown-duration hypernatremia if sodium drop is large. |
| 48 hours | 4.3 L | 90 mL/hr | Common planning window when slower correction is needed. |
| 72 hours | 4.3 L | 60 mL/hr | May be considered when sodium is very high or duration is chronic/uncertain. |
Free water deficit formula
The calculator uses this standard equation:
| TBW factor | Common use | Clinical caution |
|---|---|---|
| 0.60 | Adult male or child estimate | May overestimate in obesity, elderly or low lean body mass. |
| 0.50 | Adult female or elderly male estimate | Still an estimate; adjust to patient context. |
| 0.45 | Elderly female estimate | May better reflect lower total body water. |
| 0.40 | Dehydrated or low lean body mass estimate | Use cautiously and reassess with serial sodium checks. |
Safety rules before using the result
- Confusion
- Seizures
- Coma
- Severe weakness
- Shock or hypotension
- Serial sodium
- Glucose
- Urine output
- Fever
- Diarrhea or osmotic diuresis
- Acute vs chronic duration
- Volume status
- Kidney function
- Route available
- Local protocol
Trusted clinical references
Use these references to verify formula logic, definition and correction safety.
StatPearls / NCBI Bookshelf: Hypernatremia MSD Manual Professional: Hypernatremia MSD Manual: Water Deficit in Hypernatremia Calculator AAFP: Diagnosis and Management of Sodium Disorders Evaluation and Management of Hypernatremia in AdultsFree Water Deficit Calculator FAQs
What is a free water deficit calculator?
It estimates how much electrolyte-free water may be needed to reduce elevated serum sodium toward a target sodium level in hypernatremia.
What formula does this calculator use?
It uses Free Water Deficit = Total Body Water Ă— [(Current Na / Target Na) − 1]. Total Body Water is estimated from body weight and a TBW factor.
When should this calculator be used?
It is most relevant when serum sodium is elevated, commonly above 145 mEq/L, and free water deficit is part of clinical planning.
Can patients use this calculator to treat themselves?
No. Hypernatremia can be serious and may require urgent medical care, lab monitoring and clinician-directed treatment.
Does the calculator include ongoing losses?
The basic formula estimates existing deficit. The optional ongoing losses field adds estimated extra liters to the planning table, but it does not replace real fluid balance monitoring.
What target sodium should I use?
Target sodium depends on the patient, current sodium, duration, symptoms and correction plan. Many calculators use 140 or 145 as a reference target, but clinicians should choose the target.
How fast should sodium be corrected?
Chronic or unknown-duration hypernatremia is commonly corrected slowly. Many protocols use conservative limits such as about 0.5 mEq/L per hour and 8–10 mEq/L per day, but clinical context matters.
Why does total body water factor matter?
The TBW factor estimates what fraction of body weight is water. It changes by sex, age, lean body mass, dehydration and body composition.
Last editorial check: June 2026. Formulas, correction targets and clinical recommendations can vary by institution and patient context.
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Important safety note
This tool gives educational guidance only. Always confirm payment portals, phone numbers, account balance, assistance rules and reconnection steps with the official water department or utility.