Anion Gap Calculator

Calculate your serum anion gap from sodium, chloride, and bicarbonate, with an optional potassium term and an albumin correction. The anion gap is a routine electrolyte calculation used to help characterise acid-base disturbances. This is an informational reference using published formulas, not a diagnostic test.

Sodium (Na⁺)

mmol/L

Chloride (Cl⁻)

mmol/L

Bicarbonate (HCO₃⁻)

mmol/L

Potassium (K⁺) (optional)

mmol/L

Albumin (optional: for correction)

g/dL

Anion Gap

The reference range depends on whether potassium is included and on your lab's method (modern analysers often run a little lower). A low albumin lowers the measured anion gap, which is why the albumin-corrected value is shown when albumin is entered. Discuss your electrolyte results with a healthcare provider.

Tracking your electrolytes? See your sodium, chloride, and bicarbonate trends over time in the Health3 app.

What the Anion Gap Measures

The anion gap is a calculated number that estimates the difference between the positively charged electrolytes (cations) and negatively charged ones (anions) routinely measured in blood. Blood is electrically neutral overall, so the "gap" simply reflects the anions that a standard panel does not measure directly (such as albumin, phosphate, sulphate, and organic acids). Clinicians use it mainly to help sort out the cause of a metabolic acidosis: a raised gap points toward added acids, while a normal gap points elsewhere.

Standard formula: Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
With potassium: Anion gap = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)
Albumin correction: Corrected AG = AG + 2.5 × (4.0 − albumin g/dL)
All electrolytes in mmol/L (equivalent to mEq/L for these monovalent ions).

Published Reference Ranges

Reference ranges depend on the formula and the laboratory method. The bands below are commonly cited published ranges, not a diagnosis. Modern analysers measure chloride differently than older ones, so many labs now use a lower normal range.

Anion gap variant	Commonly cited reference range
Without potassium `Na − (Cl + HCO₃)`	Approximately 8 – 12 mmol/L (some modern methods 3 – 11)
With potassium `(Na + K) − (Cl + HCO₃)`	Approximately 12 – 16 mmol/L
Albumin-corrected	Add ~2.5 per 1 g/dL that albumin is below 4.0

Important Caveats

Low albumin masks a high gap. Albumin is the main unmeasured anion, so a low albumin lowers the apparent gap. Correcting for albumin can reveal a high gap that the raw number hides.
Method dependence. The "normal" range is not universal; always read your gap against your own laboratory's reference interval.
Context is everything. The anion gap is one piece of an acid-base assessment, never a diagnosis on its own.

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Frequently Asked Questions

What is the anion gap?

The anion gap is a calculated estimate of the difference between measured cations and anions in blood, reflecting the anions a standard panel does not measure directly (albumin, phosphate, organic acids, and others). It is calculated as sodium minus the sum of chloride and bicarbonate. Clinicians use it mainly to help characterise a metabolic acidosis.

What is a normal anion gap?

Without potassium, a commonly cited range is about 8 to 12 mmol/L, though many modern laboratory methods run lower (around 3 to 11). With potassium included, it is roughly 12 to 16 mmol/L. Ranges depend on the lab method, so always compare your value with your own laboratory reference interval. These are published reference ranges, not a diagnosis.

Should I include potassium?

Both forms are used. Many labs report the anion gap without potassium because potassium concentrations are low and relatively stable, which gives the 8 to 12 range. Including potassium gives a higher number (about 12 to 16). The potassium field here is optional; whichever you use, compare against the matching reference range.

Why correct the anion gap for albumin?

Albumin is the largest contributor to the unmeasured anions, so a low albumin lowers the apparent anion gap and can hide a true elevation. The correction adds about 2.5 mmol/L for every 1 g/dL that albumin sits below 4.0 g/dL, which can unmask a high gap in people with low albumin, such as those who are critically ill or malnourished.

Does a high anion gap mean something is wrong?

A raised anion gap can point toward a metabolic acidosis with added acids, but it is only one piece of an acid-base assessment and is not a diagnosis on its own. Many factors and lab methods affect the number. Any abnormal result should be interpreted by a clinician alongside your full clinical picture.

Medical Disclaimer: The anion gap is a routine electrolyte calculation, not a diagnosis. Reference ranges depend on whether potassium is included and on the laboratory method, and a low albumin lowers the apparent gap (hence the albumin correction). This tool is for informational and educational use only and cannot characterise an acid-base disturbance on its own. Always consult a qualified healthcare provider to interpret your electrolyte and acid-base results.

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