Corrected Sodium & Free Water Deficit Calculator (Dog & Cat)
Calculate glucose-corrected sodium and free water deficit for dogs and cats, plus a minimum safe correction time - because correcting sodium too quickly in either direction carries real neurological risk.
Correct sodium SLOWLY - in either direction.
Do not exceed ~0.5 mEq/L/hour (roughly 10-12 mEq/L per 24 hours). Correcting chronic hyponatremia too fast risks osmotic demyelination syndrome (central pontine myelinolysis); correcting hypernatremia too fast risks cerebral edema. This tool estimates volumes only - fluid choice, rate, and monitoring must be individualised with serial sodium rechecks.
Species
Glucose units
Measured sodium (mEq/L = mmol/L)
Blood glucose, mg/dL (optional)
Body weight (kg)
Target sodium (mEq/L = mmol/L)
Species selection is for display only - the free water deficit formula uses the same 0.6 × body weight estimate of total body water for both dogs and cats. Sodium is entered in mEq/L, which is numerically identical to mmol/L for this ion, so no conversion toggle is needed for it.
How to use this tool
Glucose-corrected sodium
Hyperglycemia (common in diabetic ketoacidosis) pulls water into the vascular space and dilutes measured sodium without a true free-water problem. Correct for it with: corrected Na = measured Na + 1.6 × [(glucose − 100) / 100]. Enter glucose only when it's elevated and relevant - leave it blank otherwise.
Free water deficit
Free water deficit (L) = 0.6 × body weight (kg) × [(current Na ÷ desired Na) − 1]. The 0.6 constant approximates total body water as a fraction of body weight - a commonly used clinical estimate that varies with body condition, age, and species. "Current Na" is the glucose-corrected value if glucose was entered, otherwise the raw measured value.
Correct slowly
Whichever direction you're correcting, do not exceed ~0.5 mEq/L per hour (roughly 10-12 mEq/L per 24 hours). Correcting chronic hyponatremia too fast risks osmotic demyelination syndrome; correcting hypernatremia too fast risks cerebral edema. Recheck sodium every 4-6 hours and adjust the rate accordingly.
Deficit vs maintenance
The free water deficit is given in addition to maintenance fluid needs and any ongoing losses, the same way deficit replacement is layered on top of maintenance in the IV fluid rate and dehydration calculators.
Frequently asked questions
How do you calculate free water deficit in dogs and cats?
Free water deficit (L) = 0.6 × body weight (kg) × [(current Na ÷ desired Na) − 1]. The 0.6 constant approximates total body water as a fraction of body weight. Example: a 20 kg dog with a current sodium of 165 mEq/L and a target of 145 mEq/L has a deficit of 0.6 × 20 × (165/145 − 1) ≈ 1.66 L.
Why correct sodium slowly?
Correcting chronic hyponatremia too fast can cause osmotic demyelination syndrome (central pontine myelinolysis), which is often irreversible. Correcting hypernatremia too fast can cause cerebral edema as water shifts into brain cells. The safe guideline in either direction is no faster than ~0.5 mEq/L per hour, or roughly 10-12 mEq/L per 24 hours.
What is glucose-corrected sodium and when do I need it?
Hyperglycemia pulls water into the vascular space, diluting measured sodium without a true free-water problem - most relevant in diabetic ketoacidosis. The correction is: corrected Na = measured Na + 1.6 × [(glucose − 100) / 100]. Each 100 mg/dL of glucose above normal lowers measured sodium by about 1.6 mEq/L.
Does the free water deficit replace maintenance fluids?
No. The free water deficit is given in addition to maintenance fluid requirements and any ongoing losses, the same way deficit replacement is layered on top of maintenance in fluid therapy generally - it is not a substitute for the maintenance rate.