What this calculator does
Blood is electrically neutral, so positive and negative charges must balance. A routine panel measures only some of the ions present, and the anion gap is the difference left over, which stands in for everything that was not measured.
A raised gap tells a clinician that some unmeasured acid has accumulated, which narrows the list of possible causes considerably. It is a pointer toward further investigation, not an answer in itself.
The formula
Subtract chloride and bicarbonate from sodium. The result represents the unmeasured anions in the blood, which are normally a small and fairly stable quantity.
| Term | Meaning |
|---|---|
| Anion gap | Sodium less chloride and bicarbonate, in mEq/L. |
| Reference range | Typically 8 to 12 mEq/L, though this varies with the laboratory and its method. |
| Metabolic acidosis | Excess acid in the blood, which the gap helps classify but does not itself diagnose. |
The inputs explained
| Field | What to enter |
|---|---|
| Sodium (mEq/L) | Serum sodium in mEq/L, from a standard electrolyte panel. |
| Chloride (mEq/L) | Serum chloride in mEq/L. |
| Bicarbonate (HCO3) (mEq/L) | Serum bicarbonate in mEq/L, sometimes reported as total CO2. |
When to use it
Following up an abnormal panel
The gap is a routine next step once electrolytes come back outside expected values.
Understanding your own results
Seeing how the three figures combine makes a lab report easier to follow before a consultation.
Learning acid-base interpretation
The gap is the standard entry point to classifying an acidosis in clinical teaching.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does chloride move the gap?
The same sodium and bicarbonate at four chloride levels.
| Chloride | Anion gap | Reading |
|---|---|---|
| 96 mEq/L | 20.0 mEq/L | Above the reference range: worth discussing with a clinician |
| 100 mEq/L | 16.0 mEq/L | Above the reference range: worth discussing with a clinician |
| 104 mEq/L | 12.0 mEq/L | Within the reference range |
| 110 mEq/L | 6.0 mEq/L | Below the reference range |
Questions
Should albumin be accounted for?
Often, yes. Albumin is itself a negatively charged protein, so a low albumin level lowers the apparent gap and can mask a real elevation. Many clinicians add roughly 2.5 mEq/L to the gap for every 1 g/dL that albumin sits below normal.
Does the reference range vary?
It does. Ranges depend on the laboratory and on whether potassium is included in the calculation. Always read a result against the range printed on your own report rather than a general figure.
What does a raised gap suggest?
That an unmeasured acid has accumulated. There are many possible causes, they range widely in seriousness, and distinguishing between them needs clinical assessment alongside other tests. The gap narrows the question, it does not answer it.
Can this replace a clinical assessment?
No. It is one figure calculated from three lab values, with no knowledge of your symptoms, history or other results. Any abnormal result should be discussed with the clinician who ordered the test.
For another panel value that needs adjusting before it is read, see the corrected calcium calculator. For estimated kidney function, see the creatinine clearance calculator.