Potassium Deficit & Replacement Calculator

Estimate total body potassium deficit from serum potassium and calculate the KCl replacement dose for hypokalemia. Includes IV and oral replacement guidance with safe infusion rates.

Potassium Deficit & Replacement

Estimate the total body potassium deficit from body weight and serum potassium, then review IV/oral repletion guidance.

Body weight (kg)
Serum K⁺ (mEq/L)
280mEqMild-Moderate Hypokalemia
Estimated Deficit 280 mEq
Repletion guidance

Replace the estimated deficit slowly, typically at 10–20 mEq per hour IV for mild-moderate hypokalemia, with frequent rechecking of serum potassium. Oral potassium is preferred for asymptomatic patients. Never exceed 40 mEq per hour IV, and monitor ECG and renal function during rapid replacement.

Deficit calculation
ComponentValue
Target serum K⁺ (mEq/L)4.0
Measured serum K⁺ (mEq/L)3.0
Potassium gap (mEq/L)1.0
Body weight (kg)70
Total body water fraction0.4
Estimated deficit (mEq)280
Deficit formula: (4.0 − measured serum K⁺) × weight (kg) × 0.4 × 10. The 0.4 factor is the total-body-water fraction; the ×10 scaling reflects that the serum level underestimates the true total-body deficit. The result is the estimated total body potassium deficit in mEq. This is an estimate to guide replacement, not a substitute for serial potassium measurement and clinical judgment.

Understanding Potassium Deficit and Replacement

Potassium is the dominant intracellular cation and is essential for normal neuromuscular excitability, cardiac conduction, and acid-base balance. Hypokalemia — a serum potassium below 3.5 mEq/L — is common in hospitalised patients and can arise from diuretic use, vomiting or diarrhoea, poor intake, insulin therapy, or shifts of potassium into cells. Because most of the body's potassium (roughly 98%) sits inside cells, the serum level is only a small window into the total body store, which is why a simple estimate of the deficit is clinically useful before replacement begins.

The deficit estimate multiplies the gap between the target serum potassium (4.0 mEq/L) and the measured value by the patient's weight and a total-body-water fraction of 0.4, then scales the result to reflect the total body deficit. This reflects the volume of distribution of potassium and gives a rough figure in milliequivalents for how much potassium must be replaced to restore normal levels. For a 70 kg patient with a serum potassium of 3.0 mEq/L, the estimated deficit is 280 mEq — a mild-to-moderate depletion that is usually corrected over hours to days rather than all at once.

Severity and Repletion Strategy

Hypokalemia is graded by the serum level. Values of 3.0–3.4 mEq/L are mild, 2.5–2.9 mEq/L are moderate, and below 2.5 mEq/L is severe and potentially life-threatening. Mild and moderate depletion can usually be corrected with oral potassium, which is safer and better tolerated, while severe hypokalemia or symptomatic patients may require intravenous replacement. IV potassium should be given slowly — no faster than 10–20 mEq per hour in most settings, and never above 40 mEq per hour — with cardiac monitoring and frequent rechecks of serum potassium to avoid overshoot.

Clinical Caveats

The deficit formula is an estimate, not an exact measurement. Actual requirements vary with renal function, acid-base status, and ongoing losses, and the serum level can lag behind the true intracellular deficit. Always recheck serum potassium during replacement, monitor renal function, and be cautious in patients with renal impairment or those taking potassium-sparing drugs, where the risk of hyperkalemia is higher. Use this calculator as a guide alongside clinical judgment and local protocols.

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Potassium is the dominant intracellular cation, and its serum concentration is tightly regulated because even small deviations can disrupt cardiac conduction, muscle contraction, and nerve signaling. Hypokalemia — a serum potassium below 3.5 mEq/L — is one of the most common electrolyte disturbances in hospitalized patients. Estimating the total body deficit is the first step in planning safe replacement.

Estimating the deficit

The relationship between serum potassium and total body stores is not linear, but a useful clinical estimate is that each 0.1 mEq/L drop in serum potassium below 4.0 mEq/L represents approximately 100–200 mEq of total body deficit. A patient with a serum potassium of 3.0 mEq/L therefore has a deficit on the order of 100–400 mEq. Because this is an estimate, replacement should be guided by serial serum measurements rather than a single calculation.

Choosing the replacement route

Mild hypokalemia (3.0–3.5 mEq/L) in an asymptomatic patient can usually be treated with oral potassium, which is safer and more gradual. Moderate to severe hypokalemia, or hypokalemia in a patient who cannot take oral medication, requires IV potassium. The IV route carries the risk of phlebitis and, more importantly, of hyperkalemia and arrhythmia if given too quickly, so the rate must be carefully controlled.

Safe infusion rates

The maximum peripheral IV rate is 10 mEq/h. Through a central line, rates of 20–40 mEq/h are possible but require continuous cardiac monitoring. Standard replacement is 10–20 mEq/h. Potassium should never be given as a bolus, and the concentration in peripheral IV fluid should generally not exceed 40 mEq/L to avoid vein irritation.

The magnesium connection

Hypomagnesemia is a common cause of refractory hypokalemia because magnesium is required for renal potassium conservation. If magnesium is low, potassium replacement alone will not correct the deficit. Always check and replace magnesium alongside potassium when both are low. This calculator provides an estimate to guide initial dosing, but clinical judgment, serial potassium levels, and cardiac monitoring remain essential for safe management.

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Frequently Asked Questions