Free Water Deficit
Provide weight, sex (or total body water factor), and serum sodium to estimate the free water deficit in hypernatremia.
Adjust TBW factor below if patient characteristics differ (elderly, obese, paediatric).
Typical factors: males 0.6, females 0.5, elderly 0.45–0.5, children 0.6.
Total body water
42.0 L
Free water deficit
4.1 L
How to Use This Calculator
Confirm true hypernatremia
Exclude laboratory error and check for concurrent hyperglycaemia or osmotic shifts before correcting sodium.
Estimate total body water
Select sex or enter a customised factor based on age and body composition. Multiply by weight to obtain TBW.
Replace water cautiously
Infuse hypotonic fluids gradually, targeting a sodium decline of ≤10 mEq/L per 24 hours to avoid cerebral oedema.
Formula
Total body water = Weight (kg) × TBW factor
Free water deficit = TBW × (1 − 140 ÷ Serum Na⁺)
TBW factor ≈ 0.6 (men), 0.5 (women), 0.45–0.5 (elderly).
Full Description
Hypernatremia reflects free water loss relative to sodium. Calculating the deficit estimates how much hypotonic fluid is needed to restore normonatraemia. Because rapid correction risks cerebral oedema, therapy should be slow and guided by serial sodium measurements, urine output, and ongoing losses. The calculator provides a starting point for replacement; clinical judgement and institutional protocols must guide actual management.
Frequently Asked Questions
Why target sodium 140 mEq/L?
140 mEq/L approximates normal serum sodium. Adjust the target to the patient’s baseline if chronic hypernatremia is suspected.
What fluids correct free water deficit?
Use hypotonic solutions such as D5W or 0.45% saline. Account for ongoing losses and maintenance needs separately.
How fast should sodium fall?
Do not exceed 10 mEq/L in 24 hours (0.5 mEq/L per hour) to minimise risk of cerebral oedema, especially in chronic hypernatremia.
When is the TBW factor inaccurate?
Adjust factors for obesity, cachexia, or extremes of age. Consider direct measurement or specialist input in complex cases.