Anion Gap Calculator
The anion gap is a quick acid-base check computed from three electrolytes already on every metabolic panel: sodium, chloride, and bicarbonate. It estimates the 'unmeasured' anions in your blood, and it's the standard first step when clinicians read a panel for acid-base patterns. Enter three numbers from your report to get the gap and its textbook interpretation.
From your metabolic panel
From the same panel
Reported as 'CO2' on most US panels
Enter your values above to see your results.
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The formula
Anion gap = sodium − (chloride + bicarbonate), all in mEq/L. Example: Na 140, Cl 104, HCO₃ 24 → 140 − 128 = 12. This is the modern 'without potassium' convention; the older formula adds potassium and shifts the normal range up by roughly 4. This calculator uses the without-potassium version, which is what most current references use.
Correct for albumin — the most common confounder
Albumin is the largest contributor to the 'unmeasured anions' the gap estimates, so low albumin falsely lowers the gap. The standard correction: add 2.5 mEq/L for every 1 g/dL that albumin falls below 4.0. Example: gap of 10 with albumin 2.0 → corrected gap 10 + 2.5 × 2 = 15, which reads very differently. If your albumin is on the same panel, apply this before interpreting.
What the gap actually represents
Blood is electrically neutral: measured cations (sodium) and measured anions (chloride, bicarbonate) never fully account for each other, and the difference reflects unmeasured anions — albumin, phosphate, sulfate, and organic acids. The gap rises when extra acids accumulate (ketoacids, lactate, renal retention) and falls mainly when albumin drops. It's a pattern-detection tool, not a standalone result.
Limitations
Reference ranges differ by lab method and by whether potassium is included. A single gap value is a snapshot that must be read with the full panel, symptoms, and history — the same gap can be incidental in one person and significant in another. Elevated-gap patterns can accompany serious acute conditions; treat this calculator strictly as a screening signal, not a diagnosis.
Frequently asked questions
How do I calculate the anion gap?
Anion gap = sodium − (chloride + bicarbonate), all in mEq/L from the same metabolic panel. Example: Na 140, Cl 104, HCO₃ 24 → 140 − (104 + 24) = 12 mEq/L. The calculator above does this in one step and applies the textbook reference bands.
What is a normal anion gap?
The classic textbook range is 8–12 mEq/L without potassium (the modern convention) — some sources quote up to 16 when potassium is included, and individual labs set their own intervals. Low albumin lowers the gap, so the albumin correction (add 2.5 per 1 g/dL below 4.0) matters before interpreting.
Should potassium be included in the anion gap?
Most current references exclude it: potassium's concentration is small and tightly regulated, so including it just shifts the whole range up by about 4 mEq/L without adding information. This calculator uses the without-potassium formula. If you compare against a source that includes potassium, expect their 'normal' to read roughly 12–16 mEq/L.
What does a high anion gap mean?
In textbook terms, an elevated gap indicates unmeasured acids accumulating — the high-anion-gap metabolic acidosis pattern classically linked to ketoacidosis, kidney dysfunction, lactic acid buildup, or toxin exposure. It's a screening signal, not a diagnosis, and an elevated gap — especially with symptoms — deserves prompt professional evaluation rather than self-interpretation.
Is the anion gap a diagnosis?
No. It's an arithmetic combination of three routine electrolytes used as a first-pass pattern check. Acid-base assessment uses the full picture — blood pH, symptoms, history, and often arterial blood gas testing. Always review results with a qualified healthcare professional.
Sources
- Standard anion gap conventions (8–12 mEq/L without potassium; albumin correction 2.5 per g/dL) — Tietz Textbook of Laboratory Medicine; Emmett M, Narins RG. Clinical use of the anion gap. Medicine, 1977.
- Figge J, et al. Anion gap and hypoalbuminemia. Critical Care Medicine, 1998.
This calculator is for informational purposes only and is not medical advice, diagnosis, or treatment. Reference ranges are general population guidelines — always discuss your lab results with a qualified healthcare professional.