How is the anion gap calculated?

AG = sodium − (chloride + bicarbonate), in mmol/L. An optional potassium-inclusive version is (sodium + potassium) − (chloride + bicarbonate), which shifts the normal range up.

What is the albumin-corrected anion gap?

Because albumin contributes to the gap, a low albumin can hide a raised gap. The Figge correction adds 2.5 mmol/L for every 1 g/dL the albumin sits below 4.0 g/dL. The tool shows it when you enter an albumin.

What is a normal anion gap?

It is assay-dependent, classically around 8–12 mmol/L, but many modern analysers run lower. The tool compares your result against a reference range you set for your own lab, and reports only above, within, or below.

Does the anion gap diagnose a metabolic acidosis?

No. An anion gap in isolation does not establish a metabolic acidosis. Interpret it alongside a blood gas and the full clinical picture; the tool offers no diagnosis or differential.

Do electrolyte entries leave my browser?

No. Electrolyte entries stay on this device. Print for a local worksheet only.

All tools

Free tool

Serum anion gap calculator

Compute the serum anion gap (Na − [Cl + HCO₃]), an optional potassium-inclusive variant, and the albumin-corrected anion gap, against a reference range you set for your own assay. An arithmetic worksheet for the gap maths, not a diagnosis. Electrolytes stay on this device.

Sodium (mmol/L)

Chloride (mmol/L)

Bicarbonate (mmol/L)

Include potassium (use the (Na + K) − (Cl + HCO₃) variant)

Albumin (optional, g/dL)

Albumin unit

Reference range (mmol/L)

Low

High

Reference ranges are assay-specific. Many modern analysers run lower than the classic 8–12 mmol/L. Verify against your own lab's range. The potassium-inclusive variant shifts the normal range up.

Anion gap

Na − (Cl + HCO₃)12.0 mmol/L

Measured AG

Within the reference range you set

An anion gap in isolation does not establish a metabolic acidosis.

The number above is arithmetic against the range you set. It is not a diagnosis, a differential, or a cause. Interpret it only alongside a blood gas and the full clinical picture.

Arithmetic worksheet, not a diagnosis. This tool applies a published formula to the lab values you enter. It does not measure anything, interpret the clinical picture, or suggest a diagnosis, differential, or treatment. It reports only the calculated anion gap and whether it sits above, within, or below the reference range you set. Reference ranges are assay- and laboratory-specific; confirm against your own lab's range. The albumin correction is a clinical adjustment that is not universally endorsed; some authorities question its added value. An anion gap in isolation does not establish a metabolic acidosis; interpret only alongside a blood gas and the full clinical picture. All computation runs in your browser; nothing is sent to a server.

Privacy: Electrolytes stay on this device; nothing is uploaded or logged.

Formula, the potassium-inclusive variant, and the albumin (Figge) correction reproduced from: Hamilton PK, et al. Anion Gap. StatPearls / NCBI Bookshelf NBK539757..

How the serum anion gap works

The serum anion gap (sodium minus the sum of chloride and bicarbonate) estimates the unmeasured anions in plasma and helps frame a metabolic acidosis. Because roughly 2.5 mmol/L of the apparent gap is contributed per 1 g/dL of albumin, a low albumin masks a raised gap, which the albumin correction adds back. Normal ranges are assay-specific, so this tool compares against the range you set, and reports the number only. Interpretation belongs with a blood gas and the clinical picture.

Many labs still recompute the gap by hand off a printed panel. If your diagnostic-lab workflow runs on Avinya Plus, the same arithmetic can sit on the report: build electrolytes into a lab template and let the formula engine compute the anion gap against your own reference range. The arithmetic is the engine's; the interpretation stays yours.

Frequently asked questions

How is the anion gap calculated?: AG = sodium − (chloride + bicarbonate), in mmol/L. An optional potassium-inclusive version is (sodium + potassium) − (chloride + bicarbonate), which shifts the normal range up.
What is the albumin-corrected anion gap?: Because albumin contributes to the gap, a low albumin can hide a raised gap. The Figge correction adds 2.5 mmol/L for every 1 g/dL the albumin sits below 4.0 g/dL. The tool shows it when you enter an albumin.
What is a normal anion gap?: It is assay-dependent, classically around 8–12 mmol/L, but many modern analysers run lower. The tool compares your result against a reference range you set for your own lab, and reports only above, within, or below.
Does the anion gap diagnose a metabolic acidosis?: No. An anion gap in isolation does not establish a metabolic acidosis. Interpret it alongside a blood gas and the full clinical picture; the tool offers no diagnosis or differential.
Do electrolyte entries leave my browser?: No. Electrolyte entries stay on this device. Print for a local worksheet only.

Sources

Hamilton PK, et al. Anion Gap. StatPearls / NCBI Bookshelf NBK539757.

The anion-gap formula, the potassium-inclusive variant, and the albumin (Figge) correction are reproduced from StatPearls (NBK539757). Reference ranges are assay-dependent: many modern analysers run lower than the classic 8–12 mmol/L, so the tool uses a range you set. The albumin correction is not universally endorsed. Reviewed against the source on 2026-06-29.

Keep the gap on the lab record.

In Avinya Plus you can build electrolytes into a lab template and let the formula engine compute the anion gap automatically against your own reference range. See it on a quick demo.