Anion Gap Calculator (with Albumin Correction and Delta Ratio)

Enter sodium, chloride and bicarbonate to get the serum anion gap. Add potassium and albumin to see the gap with potassium, the albumin-corrected gap and the delta ratio used to spot mixed acid–base disorders.

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Written and reviewed by the Kinase Medical Team · Last reviewed 30 September 2026

Quick Answer

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mEq/L (12–16 if K⁺ is added; ranges vary by analyser). Correct it for low albumin by adding 2.5 × (4.0 − albumin g/dL). In a high-gap acidosis, the delta ratio (AG − 12) ÷ (24 − HCO₃⁻) of 1–2 means a pure high-gap acidosis; below 1 or above 2 points to a second disorder.

For learning — not for clinical decisions. This calculator is a study aid for exam preparation. Check every result against your institution's protocols and senior advice before acting on it.

Anion gap calculator

g/L ÷ 10 = g/dL

Enter sodium, chloride and bicarbonate to see the anion gap.

How it's calculated

Anion gap (standard)
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Anion gap with potassium
AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)
Albumin-corrected anion gap
Corrected AG = AG + 2.5 × (4.0 − albumin [g/dL])
Equivalent to 0.25 mEq/L per 1 g/L of albumin. Some references use 4.5 g/dL as the normal albumin; this tool uses 4.0.
Delta ratio (Δ/Δ)
Delta ratio = (AG − 12) ÷ (24 − HCO₃⁻)
Uses the corrected AG when albumin is entered. Shown only when the gap is above 12 and bicarbonate is below 24.

The body is electrically neutral, so measured cations must equal measured plus unmeasured anions. The gap is simply the unmeasured anions (mostly albumin, plus phosphate, sulphate and organic acids) minus the unmeasured cations. When an acid such as lactic acid or a ketoacid accumulates, its hydrogen ion is buffered by bicarbonate and its anion stays behind, so bicarbonate falls and the gap widens.

How to interpret the result

Delta-ratio cut-offs from StatPearls (NBK448090). Anion gap ranges vary by laboratory.
ResultWhat it suggests
AG ≤ 12 mEq/L (after albumin correction)No excess unmeasured anion. An acidosis, if present, is normal-gap (hyperchloraemic).
AG > 12 mEq/LHigh-anion-gap metabolic acidosis (ketoacidosis, lactic acidosis, kidney failure, toxins).
Delta ratio < 1Bicarbonate has fallen more than the gap has risen — a normal-gap acidosis coexists.
Delta ratio 1–2Pure high-anion-gap metabolic acidosis.
Delta ratio > 2Gap has risen more than bicarbonate has fallen — a coexisting metabolic alkalosis or a high baseline bicarbonate.

Worked example

A patient with ketoacidosis and a low serum albumin has Na⁺ 140, Cl⁻ 100, HCO₃⁻ 10, K⁺ 4 mEq/L and albumin 2.0 g/dL.

AG = 140 − (100 + 10) = 30 mEq/L. With potassium: 144 − 110 = 34 mEq/L.

Corrected AG = 30 + 2.5 × (4.0 − 2.0) = 30 + 5 = 35 mEq/L.

Delta ratio = (35 − 12) ÷ (24 − 10) = 23 ÷ 14 ≈ 1.64 → a pure high-anion-gap metabolic acidosis, with no hidden second metabolic disorder.

Limitations and common pitfalls

  • Normal ranges differ between analysers and laboratories. Use your lab's range; the 12 and 24 used in the delta ratio are conventions, not fixed truths.
  • Low albumin lowers the gap. Without correction, a lactic acidosis in a hypoalbuminaemic ICU patient can hide behind a 'normal' anion gap.
  • Do not mix formulas: a gap calculated with potassium must be compared with a with-potassium reference range.
  • The delta ratio is a rough screening tool. It assumes each mEq of acid removes one mEq of bicarbonate, which is not exact, so borderline values need the full clinical picture and blood gas.
  • The anion gap describes a metabolic acidosis; it does not diagnose one. Confirm acidaemia and the primary disorder on an arterial or venous blood gas.

Revise the concept behind this tool: Anion gap — concept, causes and exam points →

Sources

  1. StatPearls — Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NBK448090)
  2. StatPearls — Physiology, Anion Gap (NCBI Bookshelf, NBK539757)
  3. StatPearls — Metabolic Acidosis (NCBI Bookshelf, NBK482146)

For learning — not for clinical decisions. Reference ranges and cut-offs vary between laboratories and guidelines.

Frequently Asked Questions

What is the formula for the anion gap?▼
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). Some labs add potassium: (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). Potassium is often left out because its contribution is small; if you include it, compare against a with-potassium reference range.
What is a normal anion gap?▼
Commonly quoted values are about 8–12 mEq/L without potassium and 12–16 mEq/L with potassium. Modern ion-selective analysers often give lower values (StatPearls quotes 4–12 mmol/L), so always compare with your own laboratory's reference range.
Why correct the anion gap for albumin?▼
Albumin is the main unmeasured anion. For every 1 g/dL fall in albumin below 4.0 g/dL, the gap falls by about 2.5 mEq/L. In a patient with low albumin, an uncorrected gap can look normal even when acid anions such as lactate have accumulated.
What does the delta ratio tell you?▼
The delta ratio, (AG − 12) ÷ (24 − HCO₃⁻), compares the rise in the gap with the fall in bicarbonate in a high-anion-gap acidosis. A value of 1–2 fits a pure high-gap acidosis, below 1 suggests an added normal-gap acidosis, and above 2 suggests an added metabolic alkalosis.
What causes a high anion gap metabolic acidosis?▼
The main groups are ketoacidosis (diabetic or starvation), lactic acidosis, kidney failure, and toxins such as methanol, ethylene glycol and salicylates. Each adds an unmeasured acid anion, so bicarbonate falls while chloride stays roughly the same.
What causes a normal anion gap metabolic acidosis?▼
Normal-gap (hyperchloraemic) acidosis usually comes from bicarbonate loss from the gut, as in diarrhoea, from renal tubular acidosis, or from large volumes of saline. Bicarbonate is replaced by chloride, so the gap does not change.
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