Anion Gap Calculator — AG & Corrected AG
Calculate the anion gap (Na⁺ minus Cl⁻ minus HCO₃⁻) and the albumin-corrected gap from routine serum electrolytes — essential for diagnosing metabolic acidosis and identifying its cause.
mEq/L
mEq/L
mEq/L
g/dL
Within normal range (8–16 mEq/L)
- 1
Sum of measured anions
102 + 24 (Cl⁻ + HCO₃⁻) = 126 - 2
Anion gap
140 − 126 = 14
How does this calculator work?
Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻); normal is 8–16 mEq/L. A raised gap points to unmeasured acids (MUDPILES). Correct for albumin using AG + 2.5 × (4.0 − albumin) when hypoalbuminaemia is present, as it can mask a true high-gap acidosis. Enter electrolytes and albumin to get both values instantly.
Formula
How this is calculated
The anion gap reflects the concentration of unmeasured anions in the blood. Serum electroneutrality requires that cations equal anions, but the routine panel measures only sodium (the dominant cation), chloride and bicarbonate (the dominant anions). The gap between them — AG = Na⁺ − (Cl⁻ + HCO₃⁻) — is filled by proteins (mainly albumin), phosphate, sulphate and organic acids. A normal gap is 8–16 mEq/L using the traditional formula; many labs now report a narrower normal of 8–12 mEq/L with newer analysers that measure more accurately.
A raised anion gap (>16 mEq/L) signals accumulation of unmeasured acids — the mnemonic MUDPILES covers the common causes: Methanol, Uraemia, Diabetic ketoacidosis, Propylene glycol / Paracetamol, Isoniazid / Iron, Lactic acidosis, Ethylene glycol, Salicylates. A low anion gap is rarer and is usually caused by hypoalbuminaemia, hypermagnesaemia, or lithium toxicity.
Albumin accounts for the majority of the baseline anion gap. When albumin is low (common in critically ill patients), the measured gap is artefactually depressed, masking a true high-gap acidosis. The Figge correction adds 2.5 × (4.0 − albumin) for each g/dL below normal, giving the corrected AG. This calculator uses the two-variable formula (without potassium) as used by most international guidelines; add potassium to sodium if your institution uses the three-variable version. This tool is for educational and clinical reference only — laboratory and clinical judgement must always govern patient management.
Frequently asked questions
The traditional normal range is 8–16 mEq/L using the two-variable formula (Na − Cl − HCO₃). Older literature often cited 12 ± 4 mEq/L, but modern ion-selective analysers typically return a narrower range of 8–12 mEq/L. Always compare against your laboratory's reference interval, as methodology varies.
Albumin carries a strong negative charge and normally contributes 10–12 mEq/L to the anion gap. Every 1 g/dL fall in albumin below 4.0 g/dL drops the expected gap by about 2.5 mEq/L. In hypoalbuminaemic patients (e.g. ICU, liver disease, malnutrition), the raw AG may look normal while a true high-gap acidosis exists. The corrected AG unmasks that.
The MUDPILES mnemonic covers the main causes: Methanol toxicity, Uraemia (renal failure), Diabetic ketoacidosis, Propylene glycol or paracetamol overdose, Isoniazid or iron toxicity, Lactic acidosis (sepsis, shock, ischaemia, metformin), Ethylene glycol (antifreeze), and Salicylates (aspirin overdose). Clinical context and additional tests (lactate, ketones, osmolar gap, drug screen) are needed to identify the specific cause.
Also known as
TG we-Calculate Editorial Team. (2026). Anion Gap Calculator — AG & Corrected AG [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/anion-gap-calculator
TG we-Calculate Editorial Team. "Anion Gap Calculator — AG & Corrected AG." TG we-Calculate. 2026. https://we-calculate.com/calculator/anion-gap-calculator.
TG we-Calculate Editorial Team, "Anion Gap Calculator — AG & Corrected AG," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/anion-gap-calculator
@misc{wecalculate_anion_gap_calculator, title = {Anion Gap Calculator — AG & Corrected AG}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/anion-gap-calculator}}, year = {2026}, note = {TG we-Calculate} }
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