Anion Gap Calculator
Calculate the serum anion gap from sodium, chloride and bicarbonate, with an optional potassium term. The anion gap helps classify a metabolic acidosis as high-gap or normal-gap. Enter your electrolytes in mmol/L (equivalent to mEq/L).
The anion gap formula
Anion gap = Na − (Cl + HCO₃), or (Na + K) − (Cl + HCO₃) when potassium is included. All values are in mmol/L, which equals mEq/L for these monovalent ions. The gap represents unmeasured anions, chiefly albumin.
Frequently asked questions
How is the anion gap calculated?
The standard formula is anion gap = Na − (Cl + HCO₃). If you include potassium, it becomes (Na + K) − (Cl + HCO₃). For example, Na 140, Cl 104 and HCO₃ 24 give an anion gap of 12 mmol/L.
What is a normal anion gap?
Without potassium, a normal anion gap is about 8–12 mmol/L (some labs use 3–11 with newer ion-selective analysers). When potassium is included, the normal range is roughly 12–16 mmol/L. Always check the reference interval your laboratory uses.
What does a high anion gap mean?
A high anion gap suggests a high-anion-gap metabolic acidosis from unmeasured anions — causes include lactic acidosis, diabetic or alcoholic ketoacidosis, kidney failure and certain toxins (the classic mnemonics are MUDPILES and GOLD MARK). A normal-gap acidosis points instead to bicarbonate loss, such as diarrhoea or renal tubular acidosis. This calculator flags a result above 12 mmol/L as high, or above 16 mmol/L when the potassium option is ticked. A high gap does not identify the cause by itself — a lactate, ketones, renal function and a toxicology history are what narrow it down.
Should I include potassium?
Most clinicians use the potassium-free formula, because potassium moves in a narrow range and its contribution is small. Including potassium is a valid alternative, but be sure to compare the result against the matching (higher) reference range. Ticking the potassium box here switches the normal range with it, from 8–12 mmol/L to 12–16 mmol/L. Mixing the two — reading a potassium-inclusive gap against the potassium-free range — is the commonest way to produce a spuriously high result.
Does albumin affect the anion gap?
Albumin has a large effect, because it is the main unmeasured anion the gap is measuring. A low albumin lowers the measured anion gap and can mask a true high-gap acidosis. A common adjustment adds about 2.5 mmol/L to the gap for every 1 g/dL the albumin is below 4 g/dL — interpret borderline results with that in mind.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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