Free Water Deficit Calculator
Enter adult weight, sodium values, a TBW profile, replacement fluid, replacement time, and ongoing free-water loss. The calculator estimates the deficit, fluid-equivalent volume, starting hourly rate, and modeled first-day sodium change.
Free water deficit (L)6
- Estimated total body water (L)
- 42
- Selected fluid volume for the deficit (L)
- 6
- Base replacement rate (mL/hour)
- 83.33
- Additional rate for ongoing losses (mL/hour)
- 0
- Total starting fluid rate (mL/hour)
- 83.33
- Modeled sodium after 24 hours (mmol/L)
- 152.73
- Modeled first-day sodium decrease (mmol/L)
- 7.27
- Calculation summary
- Adult male uses a 60% TBW fraction; D5W uses a 100% electrolyte-free-water fraction. The model assumes scheduled replacement is delivered evenly and entered ongoing free-water losses are fully replaced.
Adult educational cross-check only; prescribe fluids from clinical assessment, volume status, glucose, kidney function, and repeat sodium values.
Serum sodium at or above 160 mmol/L, neurologic symptoms, shock, hypotension, or severe volume depletion require urgent clinical management; isotonic resuscitation may come before free-water correction.
How to use this calculator
- Enter the adult body weight selected for the fluid plan.
- Choose the total body water profile, or select a custom percentage when a clinician has chosen one.
- Enter the current and target serum sodium values.
- Choose the replacement fluid and planned replacement time.
- Add estimated ongoing electrolyte-free water loss when it should be replaced in the starting rate.
How the free water deficit is calculated
Free water deficit estimates how much electrolyte-free water would be needed to move a hypernatremic adult from the current serum sodium toward a selected target, assuming exchangeable body sodium and potassium stay constant. The first step is choosing total body water, because the same sodium value means a larger water deficit in a person with more body water.
The calculator uses these formulas:
TBW = weight kg x TBW fraction
deficit L = max(0, TBW x (current sodium / target sodium - 1))
fluid volume L = deficit L / fluid free-water fraction
base rate mL/hour = fluid volume L x 1000 / replacement hours
ongoing-loss rate mL/hour = ongoing loss mL/day / (24 x fluid free-water fraction)
modeled Na24 = current sodium x TBW / (TBW + deficit x min(24 / replacement hours, 1))
What moves the result most
Body weight, the TBW fraction, and the sodium ratio drive the deficit. Adult male uses 60%, adult female 50%, older adult male 55%, older adult female 45%, water-depleted male 50%, and water-depleted female 40%. A custom percentage is available when the patient-specific estimate should differ from those population values.
The fluid choice changes the ordered volume and rate, not the underlying water deficit. Enteral water and D5W are treated as 100% free water after glucose metabolism. The calculator treats 0.2% sodium chloride as about 78% free water and 0.45% sodium chloride as about 50% free water, so larger fluid volumes are needed to deliver the same electrolyte-free water.
What this calculator leaves out
This is a planning cross-check, not a complete hypernatremia prescription. It does not diagnose the cause of hypernatremia, classify acute versus chronic onset, adjust for hyperglycemia, predict urine electrolyte-free water clearance, account for changing renal or gastrointestinal losses, or choose resuscitation fluid for shock. The modeled 24-hour sodium value assumes the scheduled free water is delivered evenly and that the ongoing losses entered are fully replaced.
Hypernatremia can be life-threatening. Severe sodium elevation, neurologic symptoms, hypotension, shock, or severe volume depletion require bedside clinical management and serial laboratory monitoring. In a patient with intravascular volume depletion, isotonic crystalloid is often used first to restore circulation before free-water correction is planned.
Worked example
For a 70 kg adult male with sodium 160 mmol/L and a target of 140 mmol/L, TBW is 70 x 0.60 = 42 L. The free water deficit is 42 x (160 / 140 - 1) = 6 L.
With D5W over 72 hours and no ongoing losses, the fluid volume is 6 L and the base rate is 6 x 1000 / 72 = 83.33 mL/hour. By 24 hours, one third of the deficit has been delivered, so the modeled sodium is 160 x 42 / (42 + 2) = 152.73 mmol/L, a first-day decrease of 7.27 mmol/L.
Common questions
What is the free water deficit formula?
The usual deficit estimate is total body water multiplied by the sodium ratio minus one: TBW x (current sodium / target sodium - 1). This calculator floors the result at zero when the target sodium is not below the current sodium.
Which total body water percentage should I use?
Use the profile that best matches the adult patient or enter a clinician-selected custom percentage. The preset fractions are population estimates, so edema, ascites, obesity, pregnancy, major weight change, and unusual body composition can make them inaccurate.
How much free water is in D5W, 0.45% saline, and 0.2% saline?
This calculator treats enteral water and D5W as 100% free water after glucose metabolism, 0.2% sodium chloride as about 78%, and 0.45% sodium chloride as about 50%. These are planning approximations and do not replace a full fluid and electrolyte prescription.
Why must ongoing losses be added?
The deficit only describes the water already missing at the time of the sodium measurement. Ongoing urine, stool, drain, fever, respiratory, or insensible losses can keep sodium from falling as predicted unless they are estimated and replaced.
Why can measured sodium differ from the modeled value?
Real sodium movement depends on changing intake, urine electrolyte-free water clearance, kidney function, glucose, potassium, sodium gains or losses, and laboratory timing. The model assumes the scheduled free water is delivered evenly and does not update those physiologic variables.
Should hyperglycemia-corrected sodium be used?
Hyperglycemia can lower measured sodium by shifting water into the extracellular space. Clinicians often review corrected sodium before planning dysnatremia therapy, especially when glucose is markedly elevated.