Looking for more details? Read our comprehensive Free Water Deficit guide
What is Free Water Deficit?
Hypernatremia generally indicates a deficit of free water relative to total body sodium. The Free Water Deficit calculator estimates the volume of water required to normalize serum sodium concentration.
Formula
Where TBW = Total Body Water = Weight (kg) × Correction Factor.
Correction Factors:
- Child: 0.6
- Adult Male: 0.6
- Adult Female: 0.5
- Elderly Male: 0.5
- Elderly Female: 0.45
Important Limitations
The calculation assumes the patient's sodium deficit is solely due to free water loss. In patients with combined water and sodium loss (hypovolemic hypernatremia) or sodium gain, the formula might not accurately reflect the actual state, and clinical assessment should guide fluid resuscitation.
How to Use
Enter the patient's weight, sex, age (to estimate total body water), and current measured serum sodium. The tool calculates the volume of free water needed to correct the hypernatremia.
Case Example
Patient: 70 kg adult male with severe dehydration. Serum Na: 155 mEq/L.
Calculation: TBW = 70 * 0.6 = 42 L. Deficit = 42 * ((155 / 140) - 1) = 4.5 Liters.
Interpretation: The patient has a 4.5 L free water deficit. Correction should be done slowly to prevent cerebral edema.
Frequently Asked Questions
To avoid cerebral edema, serum sodium should generally not be lowered by more than 10-12 mEq/L in a 24-hour period in chronic hypernatremia.
No, the deficit represents current missing volume. Ongoing insensible and sensible losses (e.g., urine, fever) must be added to the maintenance fluid orders.