Calculate selected free-water-deficit or sodium-deficit arithmetic with visible units. This page provides arithmetic only; it does not choose a fluid, correction pace, monitoring interval, or treatment plan.
1. Convert body weight to kg when weight is entered in lb.
2. Total body water = body weight × 0.6 L/kg.
3. Free water deficit estimate = total body water x ((current sodium / target sodium) - 1).
4. Sodium deficit estimate = total body water x (target sodium - current sodium).
Rounding is for display only. This tool shows selected arithmetic only and does not set a correction rate, fluid choice, or reassessment interval.
Calculation steps will appear for verification.
A 10 kg dog has a sodium of 165 mEq/L and a selected target of 150 mEq/L.
Total body water = 10 kg x 0.6 = 6 L
Free water deficit = 6 x ((165 / 150) - 1) = 0.6 L
The planning estimate is 0.6 L of free water deficit, which still needs a separate decision about pacing, fluids, and monitoring.
Estimate the answer before revealing it: a 4 kg cat has sodium 120 mEq/L and target 130 mEq/L in sodium-deficit mode.
Total body water = 4 kg x 0.6 = 2.4 L.
Sodium deficit estimate = 2.4 x (130 - 120) = 24 mEq.
That figure is arithmetic only; the actual correction plan and review schedule still need clinical judgment.
Last reviewed: May 2026
References: Plumb's Veterinary Drug Handbook, Merck Veterinary Manual, BSAVA Formulary, AAHA Guidelines. Verify against current references before clinical use.