Anion Gap Calculator

Calculate the serum anion gap from sodium, chloride, and bicarbonate — with optional potassium and the albumin-corrected gap used when albumin is low.

With / Without PotassiumAlbumin CorrectionRange Interpretation

Clinical disclaimer: Educational tool. Anion gap interpretation depends on the clinical picture, the lab’s own reference range, and often a full blood-gas work-up — it does not diagnose anything on its own.

Electrolytes (mEq/L)

Values from the same basic metabolic panel; albumin is optional but recommended when low

Sodium (Na⁺)

Chloride (Cl⁻)

Bicarbonate (HCO₃⁻)

Albumin, g/dL (optional)

What the Anion Gap Actually Measures

Serum is electrically neutral — total cations equal total anions. The gap is the part of that balance the basic metabolic panel doesn’t see: AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ unmeasured anions (albumin, phosphate, sulfate, organic acids) minus unmeasured cations (K⁺, Ca²⁺, Mg²⁺). Worked example: Na 140, Cl 104, HCO₃ 14 → 140 − 118 = 22 mEq/L. The elevation says an acid the panel can’t name — lactate, ketoacids, a toxin metabolite — has consumed bicarbonate and left its anion behind. That single subtraction is why the gap is the standard first split in every metabolic-acidosis workup.

Worked Lab Examples

Na	Cl	HCO₃	Albumin	Gap	Reading
140	104	24	4.0	12	Normal panel
140	104	14	4.0	22	High AG — e.g. DKA, lactic acidosis
138	110	16	4.0	12	Normal-gap acidosis (hyperchloremic)
137	102	24	2.0	11 → 16 corrected	Hidden high gap unmasked by albumin
132	101	10	3.0	21 → 23.5 corrected	High AG, worse than it looks

All values mEq/L except albumin (g/dL); corrected gap = AG + 2.5 × (4.0 − albumin).

High Gap vs. Normal Gap: The Two Acidosis Families

An elevated gap points to added acid — MUDPILES territory (methanol, uremia, ketoacidosis, propylene glycol, isoniazid, lactic acidosis — the most common by far — ethylene glycol, salicylates). A normal-gap (hyperchloremic) acidosis instead means bicarbonate loss with chloride retention: diarrhea and renal tubular acidosis lead the HARDASS list, and large-volume normal saline does it iatrogenically. The distinction changes the next test ordered: high gap → lactate, ketones, osmolal gap, toxin screen; normal gap → urine anion gap to separate gut from kidney losses.

The Albumin Trap

Albumin contributes most of the normal gap, so every 1 g/dL below 4.0 lowers the measured gap by ≈2.5 mEq/L. In an ICU patient with albumin 2.0, a “normal” gap of 11 is really 11 + 2.5 × 2 = 16 — elevated. Figge and colleagues showed uncorrected gaps miss a substantial fraction of elevated gaps in hypoalbuminemic patients, which is why this calculator asks for albumin whenever it’s available. Pair the result with our creatinine clearance and eGFR tools when the workup turns renal.

Frequently Asked Questions

What is the anion gap formula?

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), all in mEq/L. Example: Na 140, Cl 104, HCO₃ 14 → 140 − 118 = 22 mEq/L, an elevated gap. The variant that includes potassium, (Na + K) − (Cl + HCO₃), runs about 4 mEq/L higher.

What is a normal anion gap?

Roughly 8–12 mEq/L without potassium (12–16 with it), but the range is analyzer-dependent — modern ion-selective electrodes read chloride higher, and some labs quote 3–11 mEq/L. Always interpret against the reference range printed on the lab report.

What causes a high anion gap?

Unmeasured acids. The classic MUDPILES list: Methanol, Uremia, Diabetic (and alcoholic/starvation) ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. Lactic acidosis and ketoacidosis are by far the most common in practice.

Why correct the anion gap for albumin?

Albumin is the largest unmeasured anion, so low albumin shrinks the gap and can hide an acidosis. Corrected AG = AG + 2.5 × (4.0 − albumin g/dL). A gap of 11 with albumin 2.0 corrects to 16 — elevated, despite the 'normal' raw value. This matters in ICU and chronically ill patients, where hypoalbuminemia is routine.

What is a normal anion gap (hyperchloremic) metabolic acidosis?

Acidosis with a normal gap means bicarbonate was lost and replaced by chloride rather than by an unmeasured acid. Common causes follow the HARDASS mnemonic: Hyperalimentation, Addison's disease, Renal tubular acidosis, Diarrhea, Acetazolamide, Spironolactone, Saline infusion — with diarrhea and RTA the classics.

What does a low anion gap mean?

It's uncommon and usually an artifact: hypoalbuminemia (most frequent), lab error, or bromide/lithium interference. A persistently low gap can be a clue to multiple myeloma, where cationic paraproteins offset the usual unmeasured anions.

Creatinine Clearance Calculator

Estimate kidney function using the Cockcroft–Gault equation with unit toggles and BMI note.

eGFR Calculator

Calculate estimated Glomerular Filtration Rate to assess kidney function using CKD-EPI and MDRD formulas.

Corrected Calcium Calculator

Correct serum calcium for albumin with the Payne formula, in US (mg/dL) or SI (mmol/L) units, with normal-range interpretation.

Disclaimer: This calculator is for general educational purposes only and is not medical advice. It does not diagnose, treat, or replace consultation with a qualified healthcare professional. Always discuss your results with your doctor.