Anion Gap Calculator (Albumin-Corrected + Delta Ratio)

The anion gap estimates the unmeasured anions in serum by comparing the main measured cation (sodium) with the main measured anions (chloride and bicarbonate). It is the fastest bedside screen for the cause of a metabolic acidosis. This calculator uses the formulas published by StatPearls (NCBI Bookshelf) and Medscape.

1. Anion gap. Without potassium the gap is Na − (Cl + HCO₃), with a normal range of roughly 4–12 mmol/L. Including potassium gives (Na + K) − (Cl + HCO₃), which raises the reference range to about 8–16 mmol/L. The tool shifts the range automatically when you toggle potassium.
2. Albumin correction. Albumin is the dominant unmeasured anion, so a low albumin lowers the measured gap and can mask a real acidosis. The correction adds back 2.5 × (4.0 − albumin) mmol/L for albumin measured in g/dL. A patient with albumin 2.0 g/dL has a gap read about 5 mmol/L too low.
3. Classification. The corrected gap (or the raw gap when no albumin is entered) is compared with the upper reference limit. Above it is a high-anion-gap acidosis; the tool then lists the GOLD MARK differential. Within range is normal; when bicarbonate is also low, the HARDUPS list of normal-gap (hyperchloraemic) causes is shown instead.
4. Delta ratio. Only meaningful when the gap is high, the delta ratio is (corrected gap − upper limit) ÷ (24 − HCO₃). It compares how far the gap has risen against how far bicarbonate has fallen. Below 0.4 points to a pure normal-gap acidosis, 0.4–1 to a combined disorder, 1–2 to a pure high-gap acidosis, and above 2 to a concurrent metabolic alkalosis. When bicarbonate is exactly 24 the ratio is undefined (division by zero), and the tool says so rather than showing a misleading number.

As an independent check the calculator also computes the delta gap (corrected bicarbonate = measured HCO₃ + the rise in the gap). Because the delta ratio equals 1 + (delta gap ÷ ΔHCO₃), the two methods are algebraically the same and point to the same mixed disorder — a corrected bicarbonate above 26 mmol/L flags an added alkalosis, below 22 an added normal-gap acidosis.

• StatPearls (NCBI Bookshelf) — Biochemistry, Anion Gap (NBK539757)
• StatPearls (NCBI Bookshelf) — Anion Gap and Non-Anion Gap Metabolic Acidosis (NBK448090)
• Medscape — Anion Gap: Reference Range, Interpretation (article 2087291)

The formulas, reference ranges and delta-ratio bands on this page were last cross-checked against these sources on 2026-07-17. This tool is for education and does not replace clinical assessment.