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Anion Gap Calculator

Enter serum sodium, chloride and bicarbonate to calculate the anion gap instantly — with an optional potassium term, an albumin-corrected anion gap, the normal range and a plain-language interpretation.

Try a scenario:
Correct for albumin (optional)

Normal albumin is 4.0 g/dL (40 g/L). A low albumin lowers the measured anion gap by ~2.5 mEq/L per 1 g/dL.

Anion gap 12 mEq/L
Normal

AG = Na − (Cl + HCO₃) = 140 − (104 + 24) = 12 mEq/L

For education and clinical decision support only. This calculator is intended for students and clinicians and does not replace professional medical judgement, a full clinical assessment, or your laboratory's own reference ranges. Always confirm values and interpretation in the clinical context.

What is the anion gap?

The anion gap is a calculated value that estimates the difference between the routinely measured cations (positive ions) and anions (negative ions) in the blood. Because the body is electrically neutral, this "gap" actually reflects the unmeasured anions — mainly albumin, along with phosphate, sulfate and organic acids. It is one of the most useful bedside numbers for working out the cause of a metabolic acidosis.

Anion gap formula

The standard anion gap equation uses three routine electrolytes:

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

Some laboratories and textbooks include potassium, because it is also a measured cation:

Anion gap = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)

Potassium is often left out because its serum concentration is small and tightly regulated, so it changes the result only slightly. Whichever version you use, keep it consistent and interpret it against the matching normal range. Use the toggle above to switch between the two.

Normal anion gap range

Traditional teaching puts the normal anion gap at 8–12 mEq/L (often written as 12 ± 4) when potassium is not included, and about 12–16 mEq/L when potassium is included. Modern analysers using ion-selective electrodes tend to report chloride slightly higher, so many labs now quote a lower normal gap of roughly 3–11 mEq/L. Because of this variation, always compare a result with the reference range printed on your own laboratory report.

How to interpret the anion gap

  • High anion gap (> 12 mEq/L): suggests a high anion gap metabolic acidosis (HAGMA) from added unmeasured acids.
  • Normal anion gap: if the patient is acidotic (low HCO₃⁻) with a normal gap, this points to a normal anion gap (hyperchloraemic) metabolic acidosis (NAGMA), typically from loss of bicarbonate.
  • Low anion gap (< 8 mEq/L): less common; seen with low albumin, lithium or bromide intoxication, or paraproteins (e.g. multiple myeloma), and sometimes as a laboratory artefact.

High anion gap metabolic acidosis (HAGMA) causes

The classic mnemonics for the causes of a high anion gap metabolic acidosis are GOLDMARK (the modern list) and the older MUDPILES:

  • G — Glycols (ethylene glycol, propylene glycol)
  • O — Oxoproline (pyroglutamic acid, e.g. chronic paracetamol)
  • L — L-lactate (lactic acidosis)
  • D — D-lactate
  • M — Methanol
  • A — Aspirin (salicylates)
  • R — Renal failure (uraemia)
  • K — Ketoacidosis (diabetic, alcoholic, starvation)

MUDPILES stands for Methanol, Uraemia, Diabetic ketoacidosis, Propylene glycol/Paraldehyde, Iron/Isoniazid, Lactic acidosis, Ethylene glycol and Salicylates.

Normal anion gap metabolic acidosis (NAGMA) causes

A normal anion gap (hyperchloraemic) metabolic acidosis is usually caused by bicarbonate loss or reduced acid excretion. A common mnemonic is HARDASS:

  • H — Hyperalimentation (e.g. TPN)
  • A — Addison's disease (adrenal insufficiency)
  • R — Renal tubular acidosis
  • D — Diarrhoea
  • A — Acetazolamide (carbonic anhydrase inhibitors)
  • S — Spironolactone
  • S — Saline (large-volume normal saline resuscitation)

Corrected anion gap for albumin

Albumin is the largest contributor to the unmeasured anions, so hypoalbuminaemia lowers the anion gap and can hide a genuine high-gap acidosis. Correct for it by adding 2.5 mEq/L for each 1 g/dL that albumin sits below the normal 4.0 g/dL:

Corrected AG = Anion gap + 2.5 × (4.0 − albumin in g/dL)

For example, a measured anion gap of 10 with an albumin of 2.0 g/dL corrects to 10 + 2.5 × (4.0 − 2.0) = 15 mEq/L — a high gap that would otherwise be missed. Open the "Correct for albumin" section above to see this applied to your values.

Delta gap and delta-delta ratio

Once you have a high anion gap, the delta gap (or delta-delta ratio) checks whether a second acid–base disorder is hiding alongside it, by comparing the rise in the anion gap with the fall in bicarbonate. It is the natural next step after this calculator.

Frequently asked questions

What is the anion gap formula?

The anion gap is calculated as AG = Na⁺ − (Cl⁻ + HCO₃⁻), using serum sodium, chloride and bicarbonate in mEq/L. If potassium is included, the formula becomes AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻).

What is the normal anion gap range?

Without potassium, the normal anion gap is about 8–12 mEq/L (often quoted as 12 ± 4). When potassium is included the normal range is roughly 12–16 mEq/L. Reference ranges vary between laboratories, and the modern ion-selective-electrode normal is often lower (3–11 mEq/L).

What does a high anion gap mean?

A high anion gap (> 12 mEq/L) usually indicates a high anion gap metabolic acidosis (HAGMA) caused by unmeasured acids — remembered by the mnemonic GOLDMARK or MUDPILES (e.g. lactic acidosis, ketoacidosis, renal failure, methanol, ethylene glycol, salicylates).

Why correct the anion gap for albumin?

Albumin is the main unmeasured anion, so a low albumin lowers the measured anion gap and can mask a true acidosis. The corrected anion gap adds about 2.5 mEq/L for every 1 g/dL that albumin falls below 4.0 g/dL: Corrected AG = AG + 2.5 × (4.0 − albumin).

What is the significance of the anion gap?

The anion gap helps classify a metabolic acidosis as high-gap or normal-gap (hyperchloraemic), which narrows the differential diagnosis, and it can flag unmeasured anions or, when low, conditions such as hypoalbuminaemia or paraproteinaemia.

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