Electrolyte Replacement Calculations for Nurses

Electrolyte replacement is one of the most common nursing interventions in acute care. You'll replace potassium, magnesium, calcium, and phosphate on nearly every shift — especially in the ICU, post-surgical units, and cardiac floors.

Getting the dose, rate, and route right is critical. Too fast or too much can be just as dangerous as the deficiency itself. Let's go through each electrolyte so you know exactly what to expect and what to watch for.

Normal range: 3.5-5.0 mEq/L. Potassium is the electrolyte you'll replace most often.

Mild deficit (3.0-3.4 mEq/L): 20-40 mEq PO or IV. Moderate deficit (2.5-2.9): 40-80 mEq IV. Severe deficit (< 2.5): Up to 40 mEq/hr IV with continuous cardiac monitoring.

Max peripheral IV rate: 10 mEq/hr (it burns at higher rates — your patient will tell you). Max central line rate: 20-40 mEq/hr (requires cardiac monitoring). Max concentration for peripheral: 40 mEq/L. Always use an infusion pump — never bolus potassium. Recheck K+ 2-4 hours after replacement.

Normal range: 1.7-2.2 mg/dL. Magnesium is often the "forgotten" electrolyte, but it's critical for cardiac rhythm and neuromuscular function.

Mild deficit (1.2-1.6): 1-2 g IV over 1 hour. Moderate deficit (1.0-1.1): 2-4 g IV over 2-4 hours. Severe deficit (< 1.0): 4-6 g IV over 4-6 hours.

Key clinical pearl: if you can't correct a patient's potassium despite adequate replacement, check their magnesium. You often need to correct the mag first — hypomagnesemia causes refractory hypokalemia.

Normal ionized calcium: 4.5-5.3 mg/dL (1.12-1.32 mmol/L). Always use ionized calcium, not total calcium, for clinical decisions.

Calcium gluconate 10%: 1-2 g IV over 10-20 minutes. This is the preferred peripheral formulation. Calcium chloride 10%: 500 mg-1 g IV (central line strongly preferred — it's caustic to peripheral veins).

Important: calcium chloride has 3x more elemental calcium than calcium gluconate. They are NOT interchangeable gram-for-gram. Infuse slowly — rapid IV calcium can cause bradycardia and cardiac arrest. Never mix calcium with bicarbonate or phosphate in the same line (causes precipitation).

Normal range: 2.5-4.5 mg/dL. Phosphate depletion is common in DKA, refeeding syndrome, and chronic alcoholism.

Mild deficit (2.0-2.4): 0.16-0.32 mmol/kg IV over 6 hours. Severe deficit (< 1.5): 0.32-0.64 mmol/kg IV over 6-12 hours.

Choose sodium phosphate or potassium phosphate based on the patient's K+ level. Max rate: typically infuse over 6 hours minimum. Recheck phosphate 6 hours after replacement.