Free Water Deficit
Volume of pure water needed to correct hypernatremia. TBW × [(Current Na - Target Na) / Target Na]. Correct slowly to prevent cerebral edema.
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Max 0.5-1 mEq/L/hr Chronic = slower Add ongoing losses
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Why: Rapid correction of hypernatremia causes cerebral edema. Safe rates: 0.5-1 mEq/L/hour.
How: Estimate TBW from weight and body type. Calculate deficit. Divide by correction rate for replacement volume.
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Moderate Hypernatremia
Na 155, typical dehydration
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Severe Hypernatremia
Na 168 in dehydrated patient
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Critical Hypernatremia
Na 175 - emergency
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Diabetes Insipidus
Central DI with water loss
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Nursing Home Patient
Elderly with poor oral intake
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Osmotic Diuresis
DKA with hypernatremia
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Clinical Scenarios
Click a scenario to load values for common hypernatremia presentations:
Moderate Hypernatremia
Na 155, typical dehydration
Adult patient with inadequate oral intake, Na 155 mEq/L
Severe Hypernatremia
Na 168 in dehydrated patient
Elderly nursing home patient found unresponsive, severe dehydration
Critical Hypernatremia
Na 175 - emergency
ICU patient with Na 175, altered mental status, requires urgent but careful correction
Diabetes Insipidus
Central DI with water loss
Post-neurosurgery patient with polyuria and polydipsia, suspected central DI
Nursing Home Patient
Elderly with poor oral intake
Dementia patient with difficulty swallowing, chronic mild hypernatremia
Osmotic Diuresis
DKA with hypernatremia
Diabetic ketoacidosis patient with glucose-induced osmotic diuresis
Patient Parameters
Clinical Context
Free Water Deficit Results
2897
Free Water Deficit (mL)
MODERATE HYPERNATREMIA
1448
Day 1 Volume (mL)
60
Hourly Rate (mL/hr)
42.0
Total Body Water (L)
1
Days to Correct
Free water deficit: 2897 mL (2.90 L). Sodium needs to decrease by 10 mEq/L. Safe correction over 1 days at max 10 mEq/L per day.
Replacement Plan
- Day 1: Give 1448.3 mL of free water (D5W or 0.45% saline)
- Day 2: Give 869.0 mL + ongoing losses
- Day 3+: Continue 579.3 mL/day until target reached
- Hourly rate: ~60.3 mL/hr
- Check sodium every 4-6 hours during active correction
- Target sodium decrease: 10-12 mEq/L per 24 hours maximum
Replacement Plan Visualization
Volume Distribution
Sodium Correction Timeline
Severity Classification
Step-by-Step Calculations
Step 1: Determine Total Body Water Factor
Formula: ext{TBW} ext{Factor} ext{based} ext{on} ext{age}, ext{gender}, ext{body} ext{type}
Calculation: Male, normal = 0.6
Result: TBW Factor = 0.6
Step 2: Calculate Total Body Water
Formula: ext{TBW} = ext{Weight} ( ext{kg}) x ext{TBW} ext{Factor}
Calculation: TBW = 70 kg x 0.6
Result: 42.0 L
Step 3: Apply Free Water Deficit Formula
Formula: ext{Deficit} = ext{TBW} x ( ext{Current} ext{Na} / ext{Target} ext{Na} - 1)
Calculation: Deficit = 42.0 x (155 / 145 - 1)
Result: 2.90 L
Step 4: Convert to Milliliters
Formula: ext{Deficit} ( ext{mL}) = ext{Deficit} (L) x 1000
Calculation: Deficit = 2.90 x 1000
Result: 2897 mL
Step 5: Calculate Safe Correction Timeline
Formula: ext{Days} ext{needed} = ext{Sodium} ext{Change} / 10 ext{mEq}/L ext{per} ext{day}
Calculation: Days = 10 mEq/L / 10 mEq/L per day
Result: 1 days for safe correction
Hypernatremia Severity Classification
| Severity | Sodium Range | Symptoms | Management |
|---|---|---|---|
| Mild | 145-150 mEq/L | Thirst, mild confusion | Oral free water if able, monitor |
| Moderate | 150-160 mEq/L | Lethargy, irritability, weakness | IV free water (D5W/0.45% NS), q6h Na checks |
| Severe | 160-170 mEq/L | Altered mental status, hyperreflexia | ICU, IV free water, q4h Na checks |
| Critical | >170 mEq/L | Seizures, coma, high mortality | ICU, careful correction, q2h monitoring |
Total Body Water Factors
| Patient Type | Male TBW Factor | Female TBW Factor | Notes |
|---|---|---|---|
| Normal Adult | 0.60 | 0.50 | Standard adult values |
| Elderly (>65 years) | 0.50 | 0.45 | Reduced muscle mass, increased fat |
| Obese | 0.50 | 0.40 | Fat has lower water content |
| Pediatric | 0.60 | 0.60 | Higher water content in children |
What is Free Water Deficit?
Free water deficit is the amount of electrolyte-free water that must be replaced to correct hypernatremia and restore normal serum sodium concentration. This calculation is essential for treating dehydration, diabetes insipidus, and other causes of elevated sodium.
Water Deficit
Hypernatremia results from water loss exceeding sodium loss or from inadequate water intake. The deficit represents the volume of pure water needed to dilute sodium to normal.
Brain Protection
In chronic hypernatremia, brain cells adapt by generating idiogenic osmoles. Rapid correction can cause cerebral edema. Safe correction is 10-12 mEq/L per 24 hours.
Total Body Water
TBW varies by age, gender, and body composition. Accurate TBW estimation is crucial for calculating the correct replacement volume.
How to Correct Hypernatremia
Step-by-Step Approach
- 1
Assess Volume Status
Determine if hypovolemic, euvolemic, or hypervolemic hypernatremia
- 2
Identify the Cause
Dehydration, diabetes insipidus, osmotic diuresis, GI losses, etc.
- 3
Calculate Free Water Deficit
Use the formula: TBW x (Current Na / Target Na - 1)
- 4
Account for Ongoing Losses
Add estimated insensible losses, urine output, and other losses
- 5
Choose Appropriate Fluid
D5W (pure free water) or 0.45% saline (half free water)
- 6
Monitor and Adjust
Check sodium every 4-6 hours, adjust rate to achieve 10 mEq/L per day max
When to Use This Calculator
Dehydration
Elderly, debilitated, or ICU patients with inadequate water intake and elevated sodium.
Diabetes Insipidus
Central or nephrogenic DI causing massive water losses and hypernatremia.
Osmotic Diuresis
DKA, HHS, or mannitol therapy causing water losses exceeding sodium losses.
Burns
Severe burns with massive insensible water losses through damaged skin.
Fever/Infection
Increased insensible losses from fever, tachypnea, and inadequate intake.
Nursing Home Patients
Chronic hypernatremia from impaired thirst, dementia, or difficulty swallowing.
Key Formulas
1. Free Water Deficit
This gives the electrolyte-free water needed to correct hypernatremia
2. Total Body Water
TBW Factor: Men 0.6, Women 0.5, Elderly 0.5/0.45, Obese 0.5/0.4
3. Total Replacement Needed
Must account for insensible losses (600-900 mL/day) and any additional losses
4. Safe Correction Rate
Faster correction risks cerebral edema as brain cells cannot rapidly lose idiogenic osmoles
Frequently Asked Questions
Why can't I correct hypernatremia quickly?
In chronic hypernatremia (>48 hours), brain cells adapt by generating idiogenic osmoles (organic compounds like taurine, glutamine). These osmoles help maintain brain cell volume but cannot be rapidly eliminated. If you lower sodium too quickly, water will rush into brain cells causing cerebral edema, seizures, and potentially death.
What fluid should I use for replacement?
D5W provides 100% free water and is ideal for pure water deficit. 0.45% saline provides ~50% free water and is useful when some sodium replacement is also needed. 0.9% NS provides no free water but may be needed initially for volume resuscitation in hypovolemic hypernatremia. Oral water is preferred if the patient can drink.
How do I account for ongoing losses?
Insensible losses are typically 600-900 mL/day (higher with fever, tachypnea). Urine output varies - in diabetes insipidus it can be massive (5-15 L/day). GI losses from diarrhea or vomiting must be estimated. Add all ongoing losses to the free water deficit to calculate total daily replacement needs.
What's the difference between central and nephrogenic DI?
Central diabetes insipidus is caused by inadequate ADH production (from pituitary surgery, trauma, tumors) and responds to desmopressin (DDAVP). Nephrogenic DI is caused by kidney resistance to ADH (from lithium, hypercalcemia, chronic kidney disease) and does not respond to DDAVP. Treatment for nephrogenic DI includes thiazides, NSAIDs, and low-sodium diet.
When is acute hypernatremia correction faster?
If hypernatremia developed in <24 hours (acute), faster correction is safe because brain cells have not yet accumulated idiogenic osmoles. Correction can proceed at 1 mEq/L per hour. However, the timing is often uncertain, so when in doubt, treat as chronic and correct slowly at 10-12 mEq/L per 24 hours.
Clinical Pearls
Treat the Cause
Free water replacement treats the symptom, not the cause. Always identify and address the underlying etiology: desmopressin for central DI, stopping offending medications, treating infections, ensuring adequate oral intake.
Volume Before Free Water
In hypovolemic hypernatremia, restore intravascular volume first with NS or LR, then switch to hypotonic fluids for free water replacement. Hemodynamic stability takes priority over sodium correction.
Urine Output Monitoring
In diabetes insipidus, urine output can exceed 10 L/day. If urine output exceeds replacement rate, sodium will continue to rise. Match input to output plus deficit replacement, or use DDAVP to reduce losses.
Cerebral Edema Warning Signs
If sodium drops too fast (correcting faster than 10-12 mEq/L per day), watch for headache, nausea, vomiting, altered mental status, and seizures. Stop free water and consider 3% saline if cerebral edema develops.
Related Calculators
Sodium Change Calculator
Predict sodium change from IV fluids
Sodium Deficit Calculator
For hyponatremia treatment
Total Body Water
Calculate TBW accurately
IV Flow Rate
Calculate infusion rates
Serum Osmolality
Calculate osmolality
Plasma Osmolality
Plasma tonicity calculation
Sodium Correction Rate
Monitor correction velocity
Drip Rate Calculator
Calculate drops per minute
For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
Slow correction prevents cerebral edema
— Nephrology
TBW ~60% male, ~50% female
— Physiology
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