Anion Gap Calculator
Serum anion gap with and without potassium, plus the albumin-corrected value, and an honest account of why the reference range differs between laboratories.
Anion Gap Calculator: with the default inputs, anion gap is 12.
mEq/L and mmol/L are the same number for sodium, chloride, bicarbonate and potassium.
Total CO₂ on most chemistry panels.
Only used for the with-potassium version. Most US textbooks leave it out.
For the albumin-corrected gap. Set the units below to match your report.
Na⁺ − (Cl⁻ + HCO₃⁻), in mEq/L.
- Albumin-corrected anion gap
- 12The Figge correction: + 2.5 mEq/L for every g/dL the albumin sits below your laboratory's normal.
- Anion gap including potassium
- 16(Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻).
- With potassium, albumin-corrected
- 16
- Size of the albumin adjustment
- 0mEq/L added to the measured gap.
- Against your laboratory's upper limit
- At or below your laboratory's upper limit of 12 mEq/L
- Albumin in g/dL
- 4
Assumptions
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement.
- Electrolytes are entered in mEq/L, which for sodium, potassium, chloride and bicarbonate is numerically the same as mmol/L.
- The albumin correction is the Figge equation as reproduced by Kraut & Madias: 2.5 mEq/L per g/dL (0.25 per g/L) of albumin below the reference value.
- The default reference albumin of 4.0 g/dL and the default upper limit of 12 mEq/L are conventional placeholders, not universal values — both belong to your own laboratory.
- The correction does not account for blood pH, which alters the charge on albumin and therefore the size of the correction factor.
| Reference | Normal anion gap (mEq/L) | Comment |
|---|---|---|
| Halperin et al., Fluid, Electrolyte and Acid-Base Physiology, 4th ed. | Mean 12 ± 2; presumed range 8–16 | Adjust for prevailing albumin |
| Rose, Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed. | 7–13; 3–11 with ion-selective electrodes | Different values reflect different laboratory methods |
| Seifter, Goldman's Cecil Medicine, 24th ed. | Approximately 10–12 | No comment on albumin correction |
| Dubose, Brenner and Rector's The Kidney, 8th ed. | Mean 9 ± 3; presumed range 3–15 | Adjust for prevailing albumin |
| Emmett, UpToDate 2013 | 7–13; 3–9 with ion-selective electrodes | Adjust for prevailing albumin |
| Oh, Henry's Clinical Diagnosis and Management by Laboratory Methods, 22nd ed. | Approximately 12; range 8–16 | Adjust for prevailing albumin |
Reproduced from Kraut & Madias, Clin J Am Soc Nephrol 2013, Table 1. The spread across standard textbooks — anywhere from 3 to 16 — is the point: this is not a number to take from a web page.
How this is worked out
The formula
Anion gap (mEq/L) = Na⁺ − (Cl⁻ + HCO₃⁻)
Including potassium = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)
Albumin-corrected (Figge equation):
cAG = anion gap + 2.5 × (normal albumin − measured albumin), albumin in g/dL
= anion gap + 0.25 × (normal albumin − measured albumin), albumin in g/L
Normal albumin is your own laboratory's reference value; 4.0 g/dL (40 g/L) is usual.Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Sodium (Na⁺)
- mEq/L and mmol/L are the same number for sodium, chloride, bicarbonate and potassium.from 0 to 220 · defaults to 140
- Chloride (Cl⁻)
- A number.from 0 to 200 · defaults to 104
- Bicarbonate (HCO₃⁻)
- Total CO₂ on most chemistry panels.from 0 to 70 · defaults to 24
- Potassium (K⁺)
- Only used for the with-potassium version. Most US textbooks leave it out.from 0 to 12 · defaults to 4
- Serum albumin
- For the albumin-corrected gap. Set the units below to match your report.from 0 to 70 · defaults to 4
- Albumin units
- Choose one of 2 options.g/dL (United States) · g/L (most other countries)
- Your laboratory's normal albumin(under More options)
- The reference value the correction pulls towards, in the same units as above. 4.0 g/dL (40 g/L) is the usual figure.from 0.1 to 70 · defaults to 4
- Your laboratory's upper limit of normal for the anion gap(under More options)
- This genuinely varies by laboratory and by method — take it from your own report, not from a web page.from 1 to 40 · defaults to 12
What you get back
- Anion gapmain answer
- Na⁺ − (Cl⁻ + HCO₃⁻), in mEq/L.
- Albumin-corrected anion gap
- The Figge correction: + 2.5 mEq/L for every g/dL the albumin sits below your laboratory's normal.
- Anion gap including potassium
- (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻).
- With potassium, albumin-corrected
- Size of the albumin adjustment
- mEq/L added to the measured gap.
- Against your laboratory's upper limit
- Albumin in g/dL
What this assumes
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement.
- Electrolytes are entered in mEq/L, which for sodium, potassium, chloride and bicarbonate is numerically the same as mmol/L.
- The albumin correction is the Figge equation as reproduced by Kraut & Madias: 2.5 mEq/L per g/dL (0.25 per g/L) of albumin below the reference value.
- The default reference albumin of 4.0 g/dL and the default upper limit of 12 mEq/L are conventional placeholders, not universal values — both belong to your own laboratory.
- The correction does not account for blood pH, which alters the charge on albumin and therefore the size of the correction factor.
About this calculator
This is an estimate for education. It does not diagnose anything and does not replace clinical judgement. The anion gap is arithmetic on a chemistry panel; what it means is a clinical judgement made with the patient in front of you.
What it is
Blood is electrically neutral: the positive charges balance the negative ones. A routine panel measures one cation in bulk (sodium) and two anions (chloride and bicarbonate), so subtracting the measured anions from the measured cation leaves a "gap" that is filled by everything the panel does not measure — albumin above all, plus phosphate, sulfate, lactate and organic acids. When an acid accumulates, its anion joins that unmeasured pool and the gap widens.
Kraut and Madias, reviewing the calculation in the Clinical Journal of the American Society of Nephrology, note that it is most often calculated as Na⁺ − (Cl⁻ + HCO₃⁻). Potassium was in the original conception and is left out of every major US textbook because the clinical swings in potassium are small — but that reasoning fails when potassium is not small. A rise from 3.5 to 6.5 mEq/L shifts the apparent gap by 3 mEq/L if potassium is omitted, so both versions are shown here.
The reference range is not a universal number
This is the part most calculators get wrong by implying a single answer. The normal range depends on how your laboratory measures chloride. Gaps from older continuous-flow analysers cluster around 12 ± 2; ion-selective electrodes report higher chloride and therefore lower gaps, with normals as low as 3–11. Across the standard textbooks in the table below the quoted normal runs anywhere from 3 to 16 mEq/L. Two consequences follow. First, take the upper limit from your own laboratory's report — the field for it is under More options. Second, because the population range spans 8–10 mEq/L, a patient whose own baseline sits at the bottom of it can add a substantial acid load and still test "normal".
Correcting for albumin
Albumin is the largest single contributor to the unmeasured anions, so hypoalbuminaemia lowers the gap and can hide a real acidosis. The Figge correction adds 2.5 mEq/L for every g/dL the albumin sits below normal (0.25 per g/L). Correcting improved the sensitivity of the gap for detecting raised lactate substantially in published series. Kraut and Madias argue the correction should be applied by the laboratory as a matter of routine rather than left to the clinician to remember.
Where it fails
A normal gap does not rule out acidosis. In several studies roughly half the patients with a lactate above 2.5 mmol/L had a gap inside the normal range, and in one of the two studies that tested the albumin correction it still left a quarter of them undetected. If lactic acidosis is a possibility, measure lactate — the gap is not a screening test for it. The correction factor also shifts with pH, which alters albumin's charge, and the Figge equation does not account for that.
Frequently asked questions
▸What is the formula for the anion gap?
Sodium minus the sum of chloride and bicarbonate, in mEq/L. Some references add potassium to the sodium; both versions are shown here.
▸What is a normal anion gap?
There is no single answer. Standard textbooks quote anywhere from 3 to 16 mEq/L, because the value depends on how the laboratory measures chloride. Ion-selective electrodes give lower gaps than older continuous-flow analysers. Use your own laboratory's range.
▸How do I correct the anion gap for albumin?
Add 2.5 mEq/L for every g/dL that albumin sits below your laboratory's normal (0.25 per g/L). Albumin is the biggest unmeasured anion, so a low albumin can mask a genuinely raised gap.
▸Does a normal anion gap rule out acidosis?
No. In published series about half of patients with a raised lactate had a gap inside the normal range. If lactic acidosis is a concern, measure lactate directly.
▸Should potassium be included?
Most US textbooks leave it out because clinical swings in potassium are small. When potassium is markedly high or low it is worth including, since the effect on the gap is one-for-one.
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