Live calculator
Free water deficit calculator
Estimate free water replacement for hypernatremic dogs and cats. Use visible arithmetic to plan correction safely.
Safety checks
- Correct hypernatremia SLOWLY — max 0.5 mEq/L/hr or 12 mEq/L/day.
- Rapid correction risks cerebral edema and seizures.
- Use 5% dextrose in water (D5W) for free water replacement.
- Monitor sodium q4–6h during correction.
- Account for ongoing sensible and insensible losses.
Basis and limits
- Formula: Free Water Deficit (L) = TBW × weight (kg) × ((current Na / desired Na) − 1).
- Dog TBW: ~0.6 × body weight.
- Cat TBW: ~0.5 × body weight.
- Scope: Estimates free water deficit only. Does not replace clinical judgment.
Formula
Free water deficit (L): TBW × weight × ((current Na / desired Na) − 1)
Correction time (hr): (current Na − desired Na) ÷ 0.5 mEq/L/hr
D5W volume (mL): deficit (L) × 1,000
Last reviewed: May 2026
References: Plumb's Veterinary Drug Handbook, Merck Veterinary Manual, BSAVA Formulary, AAHA Guidelines. Verify against current references before clinical use.