Free tool

Anion Gap Calculator (with Albumin Correction)

Q: Where is bicarbonate on my lab report?

US metabolic panels report it as 'CO2', 'carbon dioxide' or 'total CO2' in mEq/L or mmol/L (the two are identical for these ions). That value is almost entirely bicarbonate and is what the formula uses.

Q: Why is the normal anion gap different in different sources?

Because it depends on the analyzer. Older methods measured chloride lower, giving a reference range of roughly 8–16 mEq/L; modern ion-selective electrodes read chloride higher, so the gap is smaller and labs now quote about 3–11. Use the range printed on your own report.

Q: Why correct for albumin?

Albumin is the largest unmeasured anion. Each 1 g/dL fall in albumin lowers the gap by about 2.5 mEq/L, so a person with low albumin can hide a real acidosis behind a 'normal' gap. Adding 2.5 × (4.0 − albumin) restores it.

Q: Does this calculator include potassium?

No — it uses the common US convention Na − (Cl + HCO₃). Some references add potassium, which raises every value by about 4 mEq/L and shifts the reference range up by the same amount; do not compare the two conventions directly.

The short answer

Enter sodium, chloride and bicarbonate (reported as CO₂ on a metabolic panel) and the calculator returns the anion gap, Na − (Cl + HCO₃), in mEq/L; add albumin and it also shows the albumin-corrected gap. Modern analyzers put the usual range around 3–11 mEq/L. An elevated gap with a low bicarbonate points to unmeasured acids and needs prompt evaluation.

Inputs: Na · Cl · HCO₃ (· albumin)
From: BMP or CMP
Usual range: ≈ 3–11 mEq/L (lab-dependent)
Cost: Free

Sodium

mEq/L

mEq/L = mmol/L · Sodium range

Chloride

mEq/L

Chloride range

Bicarbonate (CO₂)

mEq/L

Reported as CO₂ or total carbon dioxide on a CMP · Carbon Dioxide range

Albumin (optional)

g/dL

Leave blank to skip the albumin correction · Albumin range

Enter your values to calculate.

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) · albumin-corrected = gap + 2.5 × (4.0 − albumin [g/dL])

Educational — a derived index, not a diagnosis. Nothing you enter leaves your browser.

How the Anion gap is calculated

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) · albumin-corrected = gap + 2.5 × (4.0 − albumin [g/dL])

Worked example

Sodium (mEq/L)	Chloride (mEq/L)	Bicarbonate (CO₂) (mEq/L)	Albumin (optional) (g/dL)	Anion gap (mEq/L)
140	104	24	4	12

Albumin-corrected gap: 12.0 mEq/L · Banded on: albumin-corrected gap

Falls in: Elevated.

How to read the result

Anion gap	Reads as	What it usually means
< 3	Low	A gap under about 3 mEq/L is unusual and most often a laboratory artefact or the effect of very low albumin (if uncorrected) or an excess of unmeasured cations such as a paraprotein. It usually prompts a repeat and a look at protein studies.
3 – 12	Within the usual range	Modern ion-selective analyzers put the reference interval around 3–11 mEq/L; older methods reported 8–16. Your lab's printed range is the one that applies. A normal gap does not exclude a metabolic acidosis — a low bicarbonate with a normal gap is its own pattern.
≥ 12	Elevated	An elevated gap, especially with a low bicarbonate, points to unmeasured acids — lactate, ketones (diabetic or starvation ketoacidosis), retained acids in kidney failure, or toxins such as methanol, ethylene glycol or salicylate. This is an urgent-evaluation pattern when bicarbonate is low.

Cut points reviewed by Youmna DiStefano, MD on 2026-09-26. Sources below.

Limits

The gap depends on the analyzer, so the boundaries above are approximate and the lab's own range prevails. This calculator uses the common US convention without potassium (adding K⁺ raises every value by about 4). Low albumin lowers the gap by roughly 2.5 mEq/L per 1 g/dL, which is why the corrected value is shown when albumin is entered. It is a screening arithmetic, not an acid–base diagnosis; interpreting it needs the pH and pCO₂ from a blood gas.

Sources

Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007;2:162–174. doi.org/10.2215/CJN.03020906
Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998;26:1807–1810. doi.org/10.1097/00003246-199811000-00019
Emmett M. Approach to the Patient With a Negative Anion Gap. Am J Kidney Dis 2016;67:143–150. doi.org/10.1053/j.ajkd.2015.07.024

Frequently asked questions

Where is bicarbonate on my lab report?

US metabolic panels report it as 'CO2', 'carbon dioxide' or 'total CO2' in mEq/L or mmol/L (the two are identical for these ions). That value is almost entirely bicarbonate and is what the formula uses.

Why is the normal anion gap different in different sources?

Because it depends on the analyzer. Older methods measured chloride lower, giving a reference range of roughly 8–16 mEq/L; modern ion-selective electrodes read chloride higher, so the gap is smaller and labs now quote about 3–11. Use the range printed on your own report.

Why correct for albumin?

Albumin is the largest unmeasured anion. Each 1 g/dL fall in albumin lowers the gap by about 2.5 mEq/L, so a person with low albumin can hide a real acidosis behind a 'normal' gap. Adding 2.5 × (4.0 − albumin) restores it.

Does this calculator include potassium?

No — it uses the common US convention Na − (Cl + HCO₃). Some references add potassium, which raises every value by about 4 mEq/L and shifts the reference range up by the same amount; do not compare the two conventions directly.

Which biomarkers does this tool cover?

Which tests give you these numbers?

Physician-authorized, drawn at Quest, Labcorp or BioReference. Results in 1–3 business days for most tests.