Enter the target and current serum potassium levels plus patient weight to estimate the potassium amount (KD) needed to raise serum potassium from CK to DK. The distribution constant differs by sex: 0.4 L/kg in men and 0.3 L/kg in women. Serum potassium represents less than 2% of total-body potassium stores; all formula-based results are rough estimates subject to ongoing losses and transcellular shifts.
Potassium Deficit Formula
The standard formula estimates the potassium amount required to raise serum potassium from CK to DK using an assumed volume of distribution (Vd):
KD = (DK - CK) * W * Vd
- KD = estimated potassium required (mEq); for K+, 1 mmol = 1 mEq
- DK = desired serum potassium (mEq/L); standard target is 3.5 to 4.0 mEq/L
- CK = current serum potassium (mEq/L)
- W = body weight (kg)
- Vd = volume of distribution: 0.4 L/kg in men, 0.3 L/kg in women
The Vd constant also varies with target level: 0.4 is applied when targeting 3.5 mEq/L; some sources use 0.6 when the target is 4.0 mEq/L (reflecting greater total-body depletion at lower starting values). The calculator above uses 0.4 as the default.
Hypokalemia Severity and Clinical Reference
Hypokalemia is serum potassium below 3.5 mEq/L. The table below consolidates severity, estimated total-body deficit, ECG findings, and typical replacement route for quick clinical reference. The serum-to-total-body relationship is non-linear and steepens below 2.5 mEq/L.
| Serum K (mEq/L) | Severity | Est. Total-Body Deficit | ECG Findings | Typical Replacement Route |
|---|---|---|---|---|
| 3.0 to 3.5 | Mild | 50 to 200 mEq | T-wave flattening; possible U waves | Oral (40 to 100 mEq/day in divided doses) |
| 2.5 to 3.0 | Moderate | 150 to 300 mEq | U waves prominent (V2-V4), ST depression | Oral or IV depending on symptoms |
| 2.0 to 2.5 | Severe | 300 to 500 mEq | Wide QRS, T-wave inversion, ventricular ectopy | IV (10 to 20 mEq/hr via central line) |
| Below 2.0 | Critical | 400 to 600 mEq+ | Ventricular arrhythmias, risk of fibrillation | IV with continuous cardiac monitoring |
IV potassium rate should not exceed 40 mEq/hr even in emergencies. Serum potassium should be rechecked every 4 to 6 hours during active replacement.
What is Potassium Deficit?
Potassium deficit refers to a shortage of total-body potassium, most often caused by excessive renal or gastrointestinal losses rather than inadequate intake. Because roughly 98% of the body's potassium is intracellular, serum potassium is a poor proxy for total-body stores. Transcellular shifts driven by insulin, alkalosis, catecholamines, and beta-agonists can push serum potassium down without any change in total-body content, and vice versa during acidosis or cell lysis. This is why the formula-derived KD is an approximation: it assumes the entire deficit is reflected in serum levels, which is rarely true in clinical practice.
Common Causes of Potassium Deficit
| Mechanism | Examples |
|---|---|
| Renal losses | Thiazide and loop diuretics, primary hyperaldosteronism, Bartter and Gitelman syndromes, renal tubular acidosis, cisplatin, amphotericin B |
| GI losses | Vomiting, diarrhea, chronic laxative use, villous adenoma, ileostomy output |
| Transcellular shift | Insulin administration, metabolic alkalosis, beta-2 agonists (albuterol), hypothermia, thyrotoxic periodic paralysis, DKA treatment |
| Inadequate intake | Rarely sufficient alone; potassium-free IV fluids in NPO patients, anorexia |
The Magnesium Dependency
Refractory hypokalemia that fails to correct despite adequate replacement is a cardinal sign of concurrent hypomagnesemia. Magnesium deficiency drives renal potassium wasting by impairing the ROMK channel in the distal nephron. The clinical consequence: potassium replacement is ineffective until magnesium is repleted. This is most common in patients on loop or thiazide diuretics, those with alcohol use disorder, and patients receiving cisplatin or amphotericin B. The combination of hypokalemia plus hypomagnesemia significantly raises the risk of torsades de pointes, particularly with QT-prolonging medications.
Example: A 70 kg male patient has a serum potassium of 3.0 mEq/L. Target is 4.0 mEq/L. KD = (4.0 - 3.0) x 70 x 0.4 = 28 mEq (formula estimate). The actual total-body deficit at K = 3.0 mEq/L is typically 100 to 200 mEq, illustrating how formula output substantially underestimates true depletion.
