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Anion Gap Calculator
Anion gap = Na − (Cl + HCO₃), with the albumin-corrected (Figge) gap that unmasks a hidden acidosis and the delta gap for mixed disorders — an interpretation aid, not a diagnosis. Accurate, instant and free — for United States.
What these mean:
What these mean:
Anion gap 12 mmol/L (Na − (Cl + HCO₃); normal ≈ 8–12 mmol/L (≈ 3–11 on modern analyzers)) — within the normal range. An interpretation aid, not a diagnosis — the normal range varies by lab; verify clinically.
Anion gap
12 mmol/L
Delta gap (AG − 12)
0 mmol/L
Anion gap: normal — within the 8–12 mmol/L normal range. Ranges are lab-dependent (classically 8–12, ~3–11 on modern analyzers).
An interpretation aid, not a diagnosis
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed July 2026. Figures follow the methods and sources set out in our editorial standards.
Anion gap = Na − (Cl + HCO₃); albumin correction per Figge (+2.5 per g/dL below 4.0). An interpretation aid, not a diagnosis — verify clinically.
Figge 1998 · NCBI StatPearls
Working up the same metabolic panel? See the eGFR Calculator and the Creatinine Clearance Calculator.
The anion gap formula
From sodium, chloride and bicarbonate, this tool computes the anion gap — the unmeasured anions in plasma and a core step in working up a metabolic acidosis. It adds the two refinements simple calculators skip: the albumin-corrected gap and the delta gap for mixed disorders.
Anion gap
mmol/L ≡ mEq/L
AG = Na − (Cl + HCO₃)
Normal ≈ 8–12 mmol/L (lab-dependent; ~3–11 on modern analyzers). The optional (Na + K) form raises the normal range to ≈ 12–16.
Albumin-corrected gap
Figge correction
corrected AG = AG + 2.5 × (4.0 − albumin g/dL)
Albumin is the main unmeasured anion — a low albumin hides a raised gap. The correction adds 2.5 mmol/L per 1 g/dL below 4.0.
- 1Compute the gap: 140 − (104 + 24) = 12 mmol/L — upper-normal, looks reassuring.
- 2Correct for albumin: 12 + 2.5 × (4.0 − 2.0) = 17 mmol/L — a hidden high-anion-gap acidosis the uncorrected value misses.
Albumin-corrected anion gap (Figge)
Albumin is the largest unmeasured anion in plasma, so hypoalbuminemia lowers the apparent anion gap — and can mask a real high-anion-gap acidosis. The Figge correction restores it: corrected AG = AG + 2.5 × (4.0 − albumin g/dL). A gap of 12 at an albumin of 2.0 g/dL corrects to 17 — the concrete “hidden acidosis” demo, and the reason a normal-looking gap in a critically ill patient should always be albumin-corrected.
A normal gap in a low-albumin patient can hide an acidosis
Delta gap and the delta-delta ratio
The delta gap is how far the anion gap has risen above normal (measured AG − 12). The delta-delta ratio compares that rise to the fall in bicarbonate: Δ/Δ = (AG − 12) ÷ (24 − HCO₃). A ratio below 1 suggests a coexisting normal-anion-gap (hyperchloremic) acidosis; above 2 suggests a coexisting metabolic alkalosis or a pre-existing high bicarbonate. It is the mixed-disorder reasoning simple subtraction tools never surface.
The normal range varies by lab
The classic teaching range is 8–12 mmol/L, but it is not universal. Modern analyzers with more accurate chloride measurement run lower — around 3–11 mmol/L — so a gap of 12 can be mildly high on a contemporary panel. Including potassium shifts the whole range up to roughly 12–16 mmol/L. Always read the number against your own laboratory’s reference interval, not a single textbook figure.
High vs normal anion gap acidosis (MUDPILES)
A high anion gap metabolic acidosis (HAGMA) reflects added unmeasured acid — the mnemonic MUDPILES covers Methanol, Uremia, DKA/ketoacidosis, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol and Salicylates (GOLDMARK is a modern alternative). A normal anion gap acidosis (NAGMA) instead comes from bicarbonate loss — diarrhea or renal tubular acidosis — where chloride rises to fill the gap. The gap tells you which of these two very different families to pursue.
Frequently asked questions
The anion gap is Na − (Cl + HCO₃), in mmol/L (numerically the same as mEq/L). For a panel of sodium 140, chloride 104 and bicarbonate 24, that is 140 − (104 + 24) = 12 mmol/L. Some labs add potassium: (Na + K) − (Cl + HCO₃), which raises the result and its normal range by roughly 4. It represents the unmeasured anions in plasma and is a core step in working up a metabolic acidosis.
Albumin is the largest unmeasured anion, so a low albumin lowers the apparent anion gap and can hide a real high-anion-gap acidosis. The Figge correction adds 2.5 mmol/L to the gap for every 1 g/dL that albumin sits below 4.0 g/dL: corrected AG = AG + 2.5 × (4.0 − albumin). For example, an anion gap of 12 with an albumin of 2.0 g/dL corrects to 12 + 2.5 × (4.0 − 2.0) = 17 — a raised gap the uncorrected value completely misses. This is the single most clinically important refinement and the reason a "normal" gap in a sick, hypoalbuminemic patient should always be corrected.
The normal range is lab-dependent. The classic teaching range is 8–12 mmol/L (without potassium), but modern analyzers with more accurate chloride measurement run lower, around 3–11 mmol/L. If potassium is included, the normal range shifts up to roughly 12–16 mmol/L. Because the range varies by laboratory and method, always interpret the number against your own lab’s reference interval rather than a single textbook figure.
The delta gap is the amount the anion gap has risen above normal (measured AG − 12). The delta-delta ratio compares that rise to the fall in bicarbonate: Δ/Δ = (AG − 12) ÷ (24 − HCO₃). It flags mixed acid-base disorders: a ratio below 1 suggests a coexisting normal-anion-gap (hyperchloremic) acidosis, while a ratio above 2 suggests a coexisting metabolic alkalosis or a pre-existing high bicarbonate. It is the reasoning step that turns a single number into a picture of concurrent disorders.
A high anion gap metabolic acidosis (HAGMA) reflects added unmeasured acid. The classic mnemonic MUDPILES covers Methanol, Uremia, Diabetic (and other) ketoacidosis, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol and Salicylates; a newer mnemonic, GOLDMARK, is also used. A normal anion gap acidosis (NAGMA) instead comes from bicarbonate loss — diarrhea or renal tubular acidosis — where chloride rises to fill the gap. The gap tells you which of these two very different families to pursue.
No. It is a free, private interpretation aid that runs entirely in your browser — it computes the gap, the albumin-corrected gap and the delta gap, and flags the normal range. It is not a diagnosis. The normal range varies by lab, the result depends on accurate electrolytes and albumin, and acid-base interpretation must be made in the full clinical context. Verify clinically.
Method, standards & references
Methodology: anion gap = Na − (Cl + HCO₃) in mmol/L (≡ mEq/L); the optional (Na + K) form raises the normal range by ~4. Albumin-corrected gap (Figge) = AG + 2.5 × (4.0 − albumin g/dL). Delta gap = measured AG − 12; delta-delta = (AG − 12) ÷ (24 − HCO₃). Normal range is lab-dependent — classically 8–12, ~3–11 on modern analyzers. All calculations run client-side and nothing is stored. An interpretation aid, not a diagnosis.
Cross-links
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed July 2026. Figures follow the methods and sources set out in our editorial standards.
The anion gap = Na − (Cl + HCO₃); the albumin correction adds 2.5 mmol/L per 1 g/dL of albumin below 4.0 (Figge). This is general health information for education and an interpretation aid, not a diagnosis. The normal range varies by lab (classically 8–12, ~3–11 on modern analyzers), and acid-base interpretation must be made in the full clinical context. Always verify clinically.
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