Free Water Deficit = TBW × ((Current Na / 140) − 1). Total Body Water (TBW) = Weight × 0.6 for males or 0.5 for females (use 0.5 for elderly males, 0.45 for elderly females). For example, a 70 kg male with Na 160: TBW = 42 L, deficit = 42 × (160/140 − 1) = 6 L.

Correct sodium by no more than 10–12 mEq/L per 24 hours. For acute hypernatremia (<48 hours), faster correction is possible. For chronic hypernatremia, overly rapid correction can cause cerebral edema and seizures. Aim for 1–2 mEq/L per hour initially, then slow down. Replace the calculated deficit over 48–72 hours.

Use free water (D5W or enterally via feeding tube) for pure water replacement. If the patient is also volume-depleted, isotonic saline may be needed first for hemodynamic stability, followed by hypotonic fluids (0.45% saline or D5W) for free water replacement. Always monitor sodium levels every 4–6 hours during correction.