Anion Gap Calculator

Calculate the serum anion gap from sodium, chloride and bicarbonate. You can optionally include potassium — the result clearly shows which formula was used.

mEq/L
mEq/L
mEq/L
Enter your values above — the result appears instantly. Nothing you type leaves your device.
Medical disclaimer: This calculator is intended for educational and informational purposes and does not replace professional medical advice, diagnosis, laboratory interpretation, or treatment. Laboratory methods and reference intervals vary. Always consult a qualified healthcare professional about your results.

How the Anion Gap Calculator works

Blood is electrically neutral, but routine electrolyte panels measure only some of the charged particles. The anion gap is the difference between the main measured positive ions (sodium, and sometimes potassium) and the main measured negative ions (chloride and bicarbonate). It reflects "unmeasured" anions such as albumin, phosphate and organic acids.

Clinicians use the anion gap as one clue when evaluating acid–base disorders, particularly metabolic acidosis.

Required inputs

  • Sodium (Na⁺), chloride (Cl⁻) and bicarbonate (HCO₃⁻ or total CO₂) in mEq/L (= mmol/L)
  • Optional: potassium (K⁺) in mEq/L

Formula used

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
With potassium: Anion gap = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)

Including potassium typically adds about 4 mEq/L, so a result calculated with potassium must be compared with a reference interval for that formula.

Worked example

Na⁺ 140, Cl⁻ 104, HCO₃⁻ 24 mEq/L:

  • Without K⁺: 140 − (104 + 24) = 12 mEq/L
  • With K⁺ 4.0: (140 + 4) − (104 + 24) = 16 mEq/L

Interpreting the result

Reference intervals depend on the laboratory method. With modern ion-selective electrode analyzers, an interval of about 3–11 mEq/L (without potassium) is commonly cited; older methods used about 8–16 mEq/L. Always compare with the interval of the laboratory that produced the results.

  • A high anion gap can suggest accumulation of acids (for example lactate or ketones), kidney failure or certain toxins.
  • A low anion gap is uncommon and is most often related to low albumin or laboratory error.

The anion gap alone does not diagnose metabolic acidosis — that requires blood gas analysis and clinical assessment.

Limitations

  • Low serum albumin lowers the anion gap and can mask a raised gap; clinicians sometimes apply an albumin correction.
  • Reference intervals vary widely between analyzers and laboratories.
  • Very high lipids or proteins, bromide and lithium can interfere with measurements.
  • All values should come from the same sample.

Frequently asked questions

It estimates the unmeasured anions in blood. Clinicians use it as one clue when evaluating acid–base problems such as metabolic acidosis.

Many laboratories omit potassium because its concentration is small and stable. If you include it, the result is about 4 mEq/L higher and must be compared with a matching reference interval.

It depends on the lab method. About 3–11 mEq/L (without potassium) is commonly cited for modern analyzers, but your laboratory's own reference interval should be used.

Albumin is the main unmeasured anion. When albumin is low, the anion gap falls, which can hide an otherwise elevated gap.
Medical disclaimer: This calculator is intended for educational and informational purposes and does not replace professional medical advice, diagnosis, laboratory interpretation, or treatment. Laboratory methods and reference intervals vary. Always consult a qualified healthcare professional about your results.