About the Anion Gap Calculator
The anion gap estimates the unmeasured negatively charged ions in blood by subtracting the main measured anions (chloride and bicarbonate) from the main cation (sodium). It is a cornerstone of working up metabolic acidosis: a high anion gap points towards causes such as lactic acidosis, ketoacidosis, kidney failure or certain poisonings, while a normal gap suggests bicarbonate loss or chloride gain.
Enter the values from a basic metabolic panel in mEq/L (mmol/L gives the same numbers). You can include potassium if your lab reports the gap that way, and enter serum albumin to get the albumin-corrected anion gap — important because albumin is the largest unmeasured anion, and low albumin can hide a raised gap. When bicarbonate is below 24 the delta ratio is also shown to help spot a mixed acid–base disorder.
Reference ranges vary with laboratory equipment; many modern analysers report a normal gap of roughly 3–12 mEq/L without potassium. Use your own lab’s range where possible. This tool is intended for clinicians and students and is not a diagnosis.
With the default inputs, the anion gap is 12 mEq/L. Change any value above to recalculate instantly.
How to use the anion gap calculator
- 1Enter sodium, chloride and bicarbonate from the metabolic panel.
- 2Turn on potassium only if your reference range includes it.
- 3Enter serum albumin in g/dL to get the corrected gap.
- 4Read the corrected anion gap and, if bicarbonate is low, the delta ratio.
Formula and method
Blood is electrically neutral, so measured cations minus measured anions leaves a "gap" made up of unmeasured anions — mostly albumin, plus phosphate, sulfate and organic acids. Each 1 g/dL fall in albumin below 4.0 lowers the gap by about 2.5 mEq/L, so the Figge correction adds that amount back.
The delta ratio compares how much the gap has risen with how much bicarbonate has fallen, using 12 mEq/L as the normal gap (16 if potassium is included) and 24 mEq/L as normal bicarbonate. Values between 1 and 2 fit a pure high-gap acidosis; lower values suggest an additional normal-gap acidosis and higher values a coexisting metabolic alkalosis.
- AG
- Anion gap (mEq/L)
- Na⁺, K⁺, Cl⁻, HCO₃⁻
- Serum sodium, potassium, chloride and bicarbonate (mEq/L)
- albumin
- Serum albumin (g/dL)
Worked examples
Normal panel
Na 140 − (Cl 104 + HCO₃ 24) = 12 mEq/L. With a normal albumin of 4.0 g/dL no correction is needed, and bicarbonate is normal so no delta ratio is calculated.
Acidosis with low albumin
The raw gap is 138 − 114 = 24. Albumin is 1.0 g/dL below normal, adding 2.5 for a corrected gap of 26.5. The delta ratio is (26.5 − 12) ÷ (24 − 14) = 1.45, consistent with a pure high anion gap acidosis.
Including potassium
With potassium included the gap is 140 + 4.5 − (105 + 25) = 14.5 mEq/L, inside the roughly 7–16 range used when potassium is part of the formula.
Frequently asked questions
What is a normal anion gap?+
Without potassium, many labs now use roughly 3–12 mEq/L (older analysers gave 8–16). Including potassium raises the range by about 4 mEq/L. Always check the reference range of the lab that ran the test.
What causes a high anion gap?+
Common causes of high anion gap metabolic acidosis include lactic acidosis, diabetic or alcoholic ketoacidosis, kidney failure, and toxins such as methanol, ethylene glycol and salicylates, often remembered with mnemonics like GOLDMARK.
Why correct the anion gap for albumin?+
Albumin carries most of the normal unmeasured negative charge. When albumin is low, as in critical illness or liver disease, the measured gap falls and can mask a real high-gap acidosis. Adding 2.5 per g/dL of missing albumin corrects this.
What does the delta ratio show?+
The delta ratio compares the rise in anion gap with the fall in bicarbonate. Between 1 and 2 suggests a pure high-gap acidosis; below 1 suggests a coexisting normal-gap acidosis; above 2 suggests a coexisting metabolic alkalosis.
Can the anion gap be low?+
Yes. A low gap is most often due to low albumin or lab error, but it can also occur with high levels of paraproteins (as in multiple myeloma), lithium toxicity or severe hypercalcaemia.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.