Anion Gap with Potassium Calculator
Compute the potassium-inclusive anion gap (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). When K⁺ is added, the reference range shifts upward to 10–20 mEq/L — interpret accordingly.
When and why to include potassium
The standard anion gap formula omits potassium (Na⁺ − (Cl⁻ + HCO₃⁻)), but potassium is a cation and belongs on the positive side of the electroneutrality ledger. The potassium-inclusive variant — the formula with potassium — writes the equation as (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). Because a typical K⁺ of 4 mEq/L is added to the cation side, the entire reference interval shifts upward by roughly 4 mEq/L, giving a new normal range of 10–20 mEq/L instead of the conventional 8–12.
So why do most modern labs drop K⁺? Potassium varies little across the population (3.5–5.0 mEq/L), it adds discriminating power only at the margins, and omitting it simplifies the bedside calculation. The K-inclusive version survives in older textbooks, in some European and Australian laboratories, and in pediatrics, where potassium shifts can be more clinically prominent. If your laboratory reports its reference range as 10–20 mEq/L, it is using the K⁺-inclusive formula — and you should use this calculator, not the standard one, so your interpretation matches the lab's expectation.
Worked example
Consider a patient with sodium 140, potassium 4.0, chloride 104, and bicarbonate 24 mEq/L. The K-inclusive gap is (140 + 4.0) − (104 + 24) = 144 − 128 = 16 mEq/L — squarely in the middle of the 10–20 reference range, and exactly 4 mEq/L higher than the K-free gap of 12. The arithmetic gap between the two formulas is simply the potassium value, so they always differ by exactly K⁺.
Interpreting the K-inclusive result
A K-inclusive gap above 20 mEq/L is elevated and points to the same differential as the standard high anion gap: lactic acidosis, ketoacidosis, renal failure, and toxin ingestion (the MUDPILES / GOLD MARK categories). A value below 10 mEq/L is low and suggests hypoalbuminemia or, less commonly, paraproteinemia or laboratory error. The interpretation logic is identical to the standard formula — only the reference band is shifted. Never compare a K-inclusive gap against the 8–12 mEq/L range, or you will flag every normal patient as elevated.
To convert a K-inclusive gap to the conventional gap, simply subtract the potassium value: K-inclusive AG − K⁺ = standard AG. This is useful when comparing results between laboratories that use different formulas, or when a reference text quotes the 8–12 range but your lab reports the 10–20 range.
This calculator runs entirely in your browser — no values are transmitted or stored. The reference range and interpretation logic are reviewed against StatPearls and LITFL acid–base references. Results are informational only and should always be interpreted in full clinical context by a qualified clinician, using the reference range supplied by the testing laboratory.