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Anion Gap Calculator

Find the serum anion gap.

How it is calculated

The tool subtracts the two main measured anions from the main measured cation: AG = Na − (Cl + HCO₃). Na is serum sodium, Cl is serum chloride and HCO₃ is serum bicarbonate, all in mEq/L (the same numeric value as mmol/L for these monovalent ions). Potassium is not included. The result is rounded to one decimal and labelled Low if below 8, Normal from 8 to 12 inclusive, and High above 12. Labs use different analysers and reference ranges, so treat these cut-offs as a rule of thumb and compare against your own laboratory's range.

Worked example

Take Na = 140, Cl = 100 and HCO₃ = 15 mEq/L. First add the measured anions: 100 + 15 = 115. Then subtract from sodium: 140 − 115 = 25. The tool shows 25.0 mEq/L with the status High, because 25 is well above 12. A gap this size with a low bicarbonate points toward unmeasured acids being present.

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Common mistakes

  • Adding potassium to the formula. This tool uses the version without K⁺; mixing versions shifts the result by about 4 and changes the reference range.
  • Entering bicarbonate from a blood gas and chloride from a different draw or time point; the three values should come from the same sample.
  • Mixing units, for example entering sodium in mg/dL. All three fields expect mEq/L (mmol/L).
  • Ignoring low albumin. Low albumin lowers the true gap, so a result that looks normal can hide a real elevation.
  • Treating the label as a diagnosis. It only classifies the number; interpretation depends on the clinical picture.

Sources

About this calculator

This calculator works out the serum anion gap, a quick lab math check using the formula AG = Na⁺ − (Cl⁻ + HCO₃⁻). It measures the difference between the routinely reported cation and anions in a basic metabolic panel, which normally sits in a narrow reference range.

Clinicians, ER staff, and nursing students use it to classify metabolic acidosis as high-anion-gap (think diabetic ketoacidosis, lactic acidosis, or toxic ingestions) versus normal-anion-gap (like diarrhea or renal tubular acidosis). It's one of the first calculations run when a basic metabolic panel comes back abnormal.

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Comments (1)

  • Editorial team

    The tool flags a gap above 12 as high and below 8 as low, but laboratories use their own reference ranges, and they differ with the analyser. Potassium is not part of this formula, and low albumin can hide a raised gap because albumin carries most of the unmeasured anions. For sodium, chloride and bicarbonate, mEq/L and mmol/L are the same number. A clinician interprets the result. Would an albumin-corrected gap be useful to you?

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