Anion Gap Calculator
Calculate serum anion gap and albumin-corrected anion gap from sodium, chloride, bicarbonate and albumin.
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How to use this calculator
The anion gap is sodium minus the sum of chloride and bicarbonate, a stand-in for anions a basic panel does not measure. Albumin carries negative charge, so low albumin lowers the calculated gap. The corrected value adds 2.5 mEq/L per 1 g/dL of albumin below the 4.0 g/dL reference used here (some sources use 4.4 g/dL).
- 1
Enter serum sodium, chloride, and bicarbonate values from a basic metabolic panel.
- 2
Enter the serum albumin level (the default is 4.0 g/dL, the tool’s reference albumin).
- 3
Read the anion gap and the albumin-corrected gap.
- 4
The tool compares each gap with a single 12 mEq/L figure built into it; your laboratory report’s own interval is the one to compare against.
Frequently asked questions
What formula does the anion gap calculator use?
Anion gap = sodium − (chloride + bicarbonate), with all three entered in mEq/L. The albumin-corrected value is anion gap + 2.5 × (4 − albumin), with albumin in g/dL. Potassium is not included in the calculation.
Which units does it expect?
Sodium, chloride and bicarbonate in mEq/L, which equals mmol/L for these monovalent ions, and albumin in g/dL. If your laboratory reports albumin in g/L, divide by 10 first. The tool does not convert units for you.
What do the labels mean here?
They are wording built into this tool, using one cut-off of 12 mEq/L: a gap of 12 or below is shown as “At or below 12 mEq/L” and anything higher as “Above 12 mEq/L”. Published reference intervals differ between laboratories and analysers, so your own report’s interval takes precedence over these labels.
Why does the corrected gap differ from the raw gap?
Albumin is the main unmeasured anion, so a low albumin lowers the calculated gap. The correction adds 2.5 for each 1 g/dL below 4.0. Sources differ on the reference albumin: the original Figge equation uses 4.4 g/dL, and many calculators use 4.0 as this tool does. With sodium 136, chloride 104, bicarbonate 22 and albumin 2.0 g/dL, the tool shows a raw gap of 10.0 and a corrected gap of 15.0.
What happens if albumin is blank or zero?
With albumin at 0, which is also what a cleared box becomes, the raw anion gap is still shown, but the corrected line asks you to enter an albumin value. The tool does not assume 4.0 g/dL. A sodium, chloride or bicarbonate of 0 returns an error naming that field. An albumin above 4.0 gives a corrected gap lower than the raw one, because the formula is applied in both directions.
Does a negative or very low gap mean anything?
The tool prints any number the formula produces, such as 2.0 for sodium 128, chloride 100 and bicarbonate 26, and labels it “At or below 12 mEq/L” because it is under 12. It does not flag low gaps or point to causes like laboratory error. A clinician reviews odd values.
Does the tool name a cause for a high gap?
No. It only compares the gap with 12 mEq/L. It does not identify a cause, check other results such as lactate or ketones, or compute delta-delta ratios.
Is this a diagnostic tool?
No. The labels are wording from one built-in cut-off. A qualified clinician interprets electrolytes alongside blood gas results, symptoms and history.
Anion gap calculator — raw and albumin-corrected values
What the arithmetic does
The serum anion gap is the sodium concentration minus the sum of chloride and bicarbonate. It stands in for anions that a basic metabolic panel does not measure. Because albumin carries a negative charge, a low albumin shrinks the gap, so the tool also offers a corrected figure that adds 2.5 mEq/L for every 1 g/dL that albumin is under 4.0 (the original Figge equation uses a 4.4 g/dL reference; the tool uses 4.0). Potassium is left out of the sum. The inputs are in mEq/L for the three electrolytes and g/dL for albumin, and the tool prints both gaps to one decimal place.
Four runs with fixed inputs
The rows below show the raw and corrected gap together with the labels this tool prints. The third row is the reason the corrected line exists: the raw gap of 10.0 is at or below the built-in 12, while the corrected 15.0 is above it.
| Na / Cl / HCO3 / albumin | Anion gap | Corrected gap | Tool’s corrected label |
|---|---|---|---|
| 140 / 102 / 24 / 4.0 | 14.0 | 14.0 | Above 12 mEq/L |
| 138 / 98 / 14 / 4.0 | 26.0 | 26.0 | Above 12 mEq/L |
| 136 / 104 / 22 / 2.0 | 10.0 | 15.0 | Above 12 mEq/L |
| 128 / 100 / 26 / 4.0 | 2.0 | 2.0 | At or below 12 mEq/L |
Why the first row is worth a second look
The default values (sodium 140, chloride 102, bicarbonate 24) give a gap of 14.0, which is above the tool’s limit of 12. Laboratories set different intervals, so a value just above 12 is a prompt to check your report’s interval, not a conclusion.
What the tool leaves out
It ignores potassium, lactate, ketones, osmolality and blood gas values. It will not tell you the cause of a raised gap and it does not compare the change in gap with the change in bicarbonate. Input units are fixed at mEq/L and g/dL. An albumin of 0, which is what a cleared box becomes, is not guessed at: the corrected line waits for a value, and a sodium, chloride or bicarbonate of 0 returns an error. Because the albumin correction can also subtract, an albumin above 4.0 g/dL lowers the corrected value relative to the raw one, which is worth remembering when the two lines disagree in direction.
General information only
This page is general information, not medical advice and not for clinical decisions. A qualified clinician looks at the gap with the other electrolytes, the blood gas, the patient’s medicines and the clinical picture.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →