Free Water Deficit Calculator
Estimate how much free water a hypernatremic patient is short, work out how fast you are allowed to give it back, and check whether the sodium is real before you treat it.
Enter the values
Total body water as a fraction of weight. Use the lower fractions in the elderly and in obesity.
If the glucose is high, the reported sodium is falsely low. Enter it and the calculator will correct first.
Free water deficit
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Enter valuesCheck the sodium is real before you treat it
Two things make a reported sodium misleading, and both are common in exactly the patients who present with hypernatremia.
Hyperglycemia. Glucose is osmotically active and pulls water out of cells into the plasma, diluting the sodium. So the measured sodium underestimates the true sodium. Correct by adding 1.6 mEq/L for every 100 mg/dL the glucose sits above 100. At glucose levels above roughly 400, a factor closer to 2.4 fits better. A patient in hyperosmolar hyperglycemic state with a measured sodium of 145 and a glucose of 900 has a corrected sodium near 158, and the free water deficit computed from 145 will be far too small.
A stale or hemolyzed sample, or blood drawn above a running line. If a sodium changes by more than a few points in a patient who has not been treated, redraw before you act.
How fast you are allowed to correct
In hypernatremia present for more than 48 hours, or of unknown duration, do not lower the sodium faster than about 10 to 12 mEq/L in 24 hours, and many nephrologists prefer 8 to 10.
The brain defends itself against a rising sodium by generating intracellular osmoles over a day or two. Drop the plasma sodium faster than the brain can dismantle them and water floods into brain cells. That is cerebral edema, seizures, and death. The mechanism is the mirror image of osmotic demyelination in over-rapid correction of hyponatremia, and the harm is just as real.
Genuinely acute hypernatremia, meaning clearly under 48 hours, usually iatrogenic or from a massive salt ingestion, can be corrected faster because the brain has not adapted. If you are not certain it is acute, treat it as chronic.
Practically: recheck a sodium every 4 to 6 hours while you are correcting, not once a day. The calculator gives you a target rate but the patient's response is what governs, and free water deficits computed from a formula routinely need adjusting once you see how the number actually moves.
Which fluid, and how much
The deficit above is free water, not the volume of any particular bag. What you hang depends on the patient's volume status.
Restore the circulation first with isotonic saline, then correct the water deficit. A patient who is hypotensive from volume loss needs perfusion before tonicity. Fixing the sodium in a shocked patient is the wrong order of operations.
Oral or enteral water is the best route whenever the gut works and the patient can protect their airway. It is free, it is physiologic, and it avoids a line. Otherwise 5 percent dextrose in water, which is effectively free water once the dextrose is metabolized. Watch the glucose, because a fast D5W infusion can push a patient into hyperglycemia and an osmotic diuresis that makes the hypernatremia worse.
Uncommon, and usually iatrogenic from hypertonic saline or sodium bicarbonate. Here you need free water and a loop diuretic, and sometimes dialysis.
If you are using half-normal saline rather than D5W, only half of each liter counts as free water. A patient with a 4 liter deficit needs 8 liters of half-normal saline to replace it, which is a great deal of volume. That is worth knowing before you write the order.
Do not forget ongoing losses
The deficit formula tells you what the patient is behind by right now. It says nothing about what they are still losing, and this is the single most common reason a correction stalls.
- Insensible losses run roughly 30 to 50 mL per hour in an afebrile adult, and rise substantially with fever, tachypnea and burns.
- Obligate urinary losses continue and can be enormous in diabetes insipidus or an osmotic diuresis.
- Gastrointestinal losses from diarrhea, nasogastric suction and high-output ostomies.
Add the anticipated 24 hour ongoing losses to the calculated deficit when you write the rate, then reassess with a repeat sodium rather than trusting the arithmetic.
Why the patient is hypernatremic
Hypernatremia almost always means one of two things: the patient could not get to water, or the patient could not hold on to water. Healthy people with an intact thirst mechanism and access to a cup do not become hypernatremic.
- No access to water. The intubated patient, the nursing home resident with dementia, the postoperative patient nobody offered a drink to, the infant. This is the most common cause on an inpatient service and it is largely preventable.
- Renal water loss. Central or nephrogenic diabetes insipidus, osmotic diuresis from hyperglycemia, mannitol or high protein tube feeds, and the post-obstructive or recovery-phase diuresis after acute kidney injury.
- Extrarenal water loss. Fever, burns, diarrhea, and prolonged tachypnea.
- Sodium gain. Hypertonic saline, sodium bicarbonate during a code, hypertonic feeds, and rarely a deliberate salt ingestion.
A urine osmolality and a urine sodium separate most of these in one draw. A dilute urine in the face of a high plasma sodium means the kidney is losing water inappropriately and points at diabetes insipidus.
The mistakes that hurt people
1. Correcting too fast in chronic hypernatremia, and causing cerebral edema.
2. Calculating a deficit from an uncorrected sodium in a hyperglycemic patient, and underestimating it badly.
3. Ignoring ongoing losses, then being confused when the sodium does not move.
4. Correcting tonicity before restoring perfusion in a patient who is hypotensive. Circulation first.
Related: the clinical calculators index, and common night calls for the overnight electrolyte problems.
This is a teaching tool, not a clinical decision system. The free water deficit equation is an estimate that assumes a fixed total body sodium and ignores ongoing losses. Recheck the sodium every 4 to 6 hours during correction and follow your own institution's protocols.