Interpretation
| Result | Meaning |
|---|---|
| > 0 L | Water deficit; replace gradually |
| ≤ 0 L | No deficit |
Formula
FWD (L) = TBW × (Na⁺/140 − 1). TBW = weight × 0.6 (children and men <65), 0.5 (women <65, men ≥65), 0.45 (women ≥65).
Clinical pearls
- The formula ignores ongoing losses (urine, GI, insensible); add them to the replacement plan.
- Rapid correction of chronic hypernatraemia risks cerebral oedema; acute (<48 h) hypernatraemia can be corrected faster.
- Recheck sodium every 4–6 hours early in therapy.
Examples
free water deficit na 160 70 kg 40 yo malefwd 82 year old woman 50 kg sodium 155how much free water does a 65 kg man with sodium 152 need
FAQ
How fast should hypernatraemia be corrected?
For chronic hypernatraemia, lower sodium by no more than about 10 mmol/L in 24 hours; acute hypernatraemia (hours) can be corrected more quickly.
What fraction of body weight is water?
About 60% in children and younger men, 50% in younger women and older men, and 45% in older women.