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Anion Gap Calculator

Calculate the serum anion gap from sodium, chloride and bicarbonate, with optional potassium and an albumin-corrected anion gap, and see whether it is low, normal or high.

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For information only, not medical advice. Normal anion gap ranges depend on the lab’s equipment; use your lab’s reference range when it is given. A result outside the range needs to be read by a clinician together with the full blood gas and history.

Quick Answer

Anion gap = sodium − (chloride + bicarbonate). With sodium 140, chloride 106 and bicarbonate 24 mEq/L, the gap is 140 − 130 = 10 mEq/L. Low albumin hides a high gap, so add 2.5 for every 1 g/dL that albumin is below normal: with albumin 2.0 g/dL a gap of 26 becomes 31.

Last updated: 10 November 2026 ·How we calculate this: the standard anion gap formula with Figge’s albumin correction

How This Anion Gap Calculator Works

Blood has to carry equal positive and negative charge. A basic metabolic panel measures the main positive ion (sodium) and the main negative ions (chloride and bicarbonate), but not proteins, phosphate, sulfate or organic acids. The difference is the anion gap. Enter the sodium, chloride and bicarbonate from a blood test, and optionally potassium and albumin. The calculator gives the anion gap, the gap with potassium, and the albumin-corrected gap.

Quick explanation: a high anion gap points to extra unmeasured acids in the blood.

  • Normal: about 3 to 11 mEq/L on most modern analyzers.
  • High: often seen with lactic acidosis, ketoacidosis, kidney failure or some poisonings.
  • Low: most often caused by low albumin, or a lab error.

How to Use the Anion Gap Calculator

  1. Enter sodium, chloride and bicarbonate from the same blood test.
  2. Optionally enter potassium.
  3. Optionally enter albumin and choose the normal albumin value.
  4. Press Calculate.
  5. Read the anion gap, the corrected gap and the interpretation.

Anion Gap Formulas

  • Anion gap: Na − (Cl + HCO₃)
  • With potassium: (Na + K) − (Cl + HCO₃); its normal range is about 4 mEq/L higher
  • Albumin-corrected gap: anion gap + 2.5 × (normal albumin − measured albumin in g/dL)
  • Units: mEq/L and mmol/L are the same for these single-charge ions

Figge and colleagues found that the gap falls by about 2.5 mEq/L for each 1 g/dL drop in albumin. Many references use 4.0 g/dL as normal albumin; Figge used 4.4 g/dL, so you can choose either.

Key Terms

  • Anion gap: the difference between measured positive and negative ions in the blood.
  • mEq/L: milliequivalents per liter, a unit of charge concentration.
  • Bicarbonate (HCO₃): the main buffer that neutralizes acid in the blood.
  • Albumin: the main blood protein; it carries negative charge.
  • Metabolic acidosis: too much acid or too little bicarbonate in the blood.

Worked Example

A patient has sodium 138, chloride 100 and bicarbonate 12 mEq/L, with albumin 2.0 g/dL.

  • Anion gap = 138 − (100 + 12) = 26 mEq/L, which is high
  • Albumin is 2.0 g/dL below a normal of 4.0
  • Corrected gap = 26 + 2.5 × 2.0 = 31 mEq/L, even higher

Another patient has sodium 136, chloride 110 and bicarbonate 18: the gap is 8, normal, so a low bicarbonate with a normal gap points to a normal-gap (hyperchloremic) acidosis such as diarrhea.

Real-Life Examples

  • Study: practice anion gap questions for nursing, pharmacy and medical exams.
  • Lab reports: sodium 140, chloride 106 and bicarbonate 24 give a normal gap of 10 mEq/L.
  • Low albumin: a gap of 26 with albumin 2.0 g/dL corrects to 31 mEq/L.
  • Figge correction: with albumin 4.2 g/dL and a 4.4 reference, a gap of 8 corrects to 8.5.

Anion Gap vs Corrected Anion Gap

  • Anion gap uses only sodium, chloride and bicarbonate. It is quick but misleading when albumin is low.
  • Corrected anion gap adds back the effect of low albumin, so a hidden high-gap acidosis shows up.
  • Use the corrected value whenever albumin is clearly below normal.

Albumin Correction Table

Corrected gap = measured gap + the amount shown.

Albumin (g/dL)Add with 4.0 normalAdd with 4.4 normal
4.4-1.00+0.00
4.0+0.00+1.00
3.5+1.25+2.25
3.0+2.50+3.50
2.5+3.75+4.75
2.0+5.00+6.00
1.5+6.25+7.25

What Changes the Anion Gap

Albumin is the biggest unmeasured anion, so low albumin, common in hospital patients, lowers the gap and can hide a high-gap acidosis. That is why the corrected gap matters. Lab methods also matter: newer ion-selective electrodes read chloride a little higher, which moved the normal range down from about 8 to 16 mEq/L to about 3 to 11 mEq/L.

A high anion gap is often remembered with the mnemonic GOLDMARK: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure and ketoacidosis. A normal gap with low bicarbonate suggests losses of bicarbonate, such as diarrhea or renal tubular acidosis. A very low or negative gap can come from low albumin, high calcium or magnesium, lithium, some myeloma proteins, or bromide that the analyzer reads as chloride.

Common Mistakes to Avoid

  • Ignoring low albumin. Each 1 g/dL drop lowers the gap by about 2.5.
  • Mixing ranges. The range with potassium is about 4 mEq/L higher than without it.
  • Using values from different samples. Take all electrolytes from the same blood draw.
  • Using the old 8 to 16 range on a modern analyzer. Check your lab’s reference range.
  • Reading the gap alone. It must be read with the pH, bicarbonate and the patient’s condition.

Frequently Asked Questions

About 3 to 11 mEq/L on most modern analyzers without potassium. Older methods gave about 8 to 16. Use your lab’s range.

Subtract chloride and bicarbonate from sodium: Na − (Cl + HCO₃).

Extra unmeasured acids, for example lactic acid, ketones, kidney failure or certain poisons.

Most often low albumin. Lab error, high calcium or magnesium, lithium and some myeloma proteins can also lower it.

Add 2.5 × (normal albumin − measured albumin in g/dL) to the gap.

Most labs leave it out. If you include it, the normal range is about 4 mEq/L higher.

Yes, for sodium, potassium, chloride and bicarbonate, which each carry one charge.

A memory aid for causes of a high anion gap: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure and ketoacidosis.

Divide by 10 to get g/dL, so 35 g/L is 3.5 g/dL.

References & Sources

SourceUsed ForLink
Kraut JA, Madias NE (2007), Serum anion gap: its uses and limitations in clinical medicine Clin J Am Soc Nephrol 2(1):162-74. Formula, reference ranges and causes. https://pubmed.ncbi.nlm.nih.gov/17699401/
Figge J, Jabor A, Kazda A, Fencl V (1998), Anion gap and hypoalbuminemia Crit Care Med 26(11):1807-10. About 2.5 mEq/L per 1 g/dL of albumin. https://pubmed.ncbi.nlm.nih.gov/9824071/