Serum anion gap calculator
The serum anion gap estimates the unmeasured anions in your blood by subtracting your main measured anions (chloride and bicarbonate) from your main measured cation (sodium). Clinicians use it mainly to sort out the cause of a metabolic acidosis. This tool does the arithmetic from your basic metabolic panel; a clinician interprets what the number means for you.
How it works
Take the sodium value from your metabolic panel and subtract the sum of chloride and bicarbonate (CO₂). That difference is the anion gap. It works because blood is electrically neutral, so the "gap" left after the routinely measured ions reflects anions the panel doesn't measure directly (such as albumin, phosphate, lactate, and ketoacids). A high gap points toward extra acid in the blood — commonly lactate, ketones, ingested toxins, or kidney failure. One important adjustment: albumin is itself a negatively charged anion, so a low albumin lowers the measured gap and can mask a real problem. Correct for it by adding 2.5 mEq/L to the gap for every 1 g/dL that albumin falls below 4.0 g/dL. On modern ion-selective-electrode analyzers the normal gap is lower than the textbook 8–12 many people remember — roughly 3–11 mEq/L — because these machines read chloride higher; always compare against your own lab's stated reference range.
Worked example
Suppose sodium is 140, chloride is 100, and bicarbonate is 24 mEq/L. Anion gap = 140 − (100 + 24) = 140 − 124 = 16 mEq/L, already above a typical modern normal range of 3–11. Now suppose albumin is low at 2.0 g/dL. Corrected gap = 16 + 2.5 × (4.0 − 2.0) = 16 + 5 = 21 mEq/L — meaningfully higher, and a clearer signal of a high-anion-gap metabolic acidosis than the uncorrected number suggested.
Important
Estimate and educational tool only — not medical advice, diagnosis, or a substitute for evaluation by a qualified clinician. This calculator performs arithmetic from values you enter; it does not verify them and cannot interpret your result. It is intended for adults; pediatric reference ranges differ. The "normal" anion gap depends on your laboratory's analyzer and methods — modern ion-selective-electrode analyzers typically read lower (about 3–11 mEq/L) than the older textbook 8–12, and adding potassium raises the range by about 4 mEq/L — so always compare your number against your own lab's stated reference range, not the figure shown here. The albumin correction assumes a normal albumin of 4.0 g/dL; some laboratories and references use a different anchor. An anion gap is meaningful only alongside the rest of your labs, your symptoms, and your history: a high gap does not by itself diagnose any condition, and a normal or corrected-normal gap does NOT rule out serious illness (including life-threatening acidosis). Do not use this tool to self-diagnose or to start, stop, or change any treatment. Seek emergency care immediately if you have rapid or deep breathing, confusion or drowsiness, severe or persistent vomiting, or any known or suspected poisoning or overdose. Review every lab result with a licensed healthcare professional.