Anion Gap Calculator (with Winter’s Formula and Delta Ratio)
Enter sodium, chloride and bicarbonate to get the anion gap. Add potassium, albumin and PaCO2 for the potassium-inclusive gap, the albumin-corrected gap, the delta ratio and Winter’s formula. Every formula and cutoff links to its official source.
For exam study, not for clinical use.
How it works
1. Anion gap
AG = Na+ − (Cl− + HCO3−)
With potassium: AG = (Na+ + K+) − (Cl− + HCO3−). Normal: 8 to 12 mEq/L, or 12 to 16 when potassium is included. Above the range = high anion gap. Potassium is often left out because it contributes little.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
Why normal ranges differ
Another StatPearls chapter gives a normal gap of 4 to 12 mmol/L and explains that it depends on the serum phosphate and albumin. A low gap is most often caused by low albumin, the main unmeasured anion.
Source: Biochemistry, Anion Gap (NCBI Bookshelf, NIH) [2]
2. Albumin-corrected anion gap
Corrected AG = measured AG + 2.5 × (4.0 − albumin in g/dL)
Low albumin lowers the measured gap and can hide a high anion gap metabolic acidosis. StatPearls example: a measured gap of 10 with albumin 2.0 g/dL gives a corrected gap of 15.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
3. Delta ratio (delta-delta)
Delta ratio = (corrected AG − 12) / (24 − measured HCO3−)
- 1 to 2: generally a pure high anion gap metabolic acidosis.
- Below 1: a concurrent normal anion gap (non-anion gap) metabolic acidosis.
- Above 2: an additional metabolic alkalosis or a preexisting high bicarbonate.
12 mEq/L is the upper limit of a normal gap and 24 mEq/L is the normal bicarbonate. The calculator uses the albumin-corrected gap when you enter albumin.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
4. Winter’s formula (expected PaCO2)
Expected PaCO2 = (1.5 × HCO3−) + 8 ± 2
In a metabolic acidosis, a measured PaCO2 inside this range means appropriate respiratory compensation. Higher than expected: an additional respiratory acidosis. Lower than expected: an additional respiratory alkalosis.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]; Partial Pressure of Carbon Dioxide (NCBI Bookshelf, NIH) [3]
When the calculator checks Winter’s formula
Only when the bicarbonate is below the official USMLE reference range (22 to 28 mEq/L), that is, when there is a metabolic acidosis to compensate for.
Source: USMLE Step 1 Sample Test Questions: USMLE Laboratory Values, pages 7 to 9 (USMLE program, usmle.org) [4]
High-yield for the exam
High anion gap causes (GOLD MARKeT): glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketones, toluene. Older mnemonic: CAT MUDPILES.
Normal anion gap causes: mainly diarrhea and renal tubular acidosis.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
Worked example
Na 140, Cl 100, HCO3 10 mEq/L, PaCO2 23 mm Hg.
- Anion gap = 140 − (100 + 10) = 30: high (above 12).
- Delta ratio = (30 − 12) / (24 − 10) = 18 / 14 = 1.29: between 1 and 2, a pure high anion gap metabolic acidosis.
- Winter’s formula = 1.5 × 10 + 8 = 23, range 21 to 25 mm Hg. Measured 23 is inside: appropriate respiratory compensation.
Press Example: high gap above to load these numbers.
Low albumin: Na 135, Cl 105, HCO3 20, albumin 2.0 g/dL.
- Measured gap = 135 − (105 + 20) = 10: looks normal.
- Corrected gap = 10 + 2.5 × (4.0 − 2.0) = 15: high (the StatPearls example).
- Delta ratio = (15 − 12) / (24 − 20) = 0.75: below 1, so a normal anion gap acidosis is also present.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
FAQ
What is the formula for calculating the anion gap in metabolic acidosis?
Anion gap = Na+ − (Cl− + HCO3−). Some labs add potassium: (Na+ + K+) − (Cl− + HCO3−). If albumin is low, correct it: corrected gap = gap + 2.5 × (4.0 − albumin in g/dL).
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
What number indicates a high anion gap?
A gap above 12 mEq/L (normal 8 to 12), or above 16 if potassium is included (normal 12 to 16). Correct for a low albumin first, because low albumin can hide a high gap.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
What would cause my anion gap to be high?
For the exam: acids that add unmeasured anions, such as lactate, ketones (diabetic, alcoholic or starvation ketoacidosis), methanol, ethylene glycol, aspirin (salicylates) and renal failure. The GOLD MARKeT and CAT MUDPILES mnemonics list them. If this is about your own blood test, please ask your doctor.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
Which is worse, high or low anion gap?
Any change from normal can matter, but StatPearls notes that the most clinically significant is an increased anion gap, especially with a metabolic acidosis.
Source: Biochemistry, Anion Gap (NCBI Bookshelf, NIH) [2]
What cancers show a low anion gap?
IgG myeloma: paraproteins such as IgG carry a positive charge and lower the anion gap. The most common cause of a low gap is low albumin. Lithium toxicity can also lower it.
Source: Biochemistry, Anion Gap (NCBI Bookshelf, NIH) [2]
How do I calculate the urinary anion gap?
Urine anion gap = urine Na + urine K − urine Cl (if urine pH is below 6.5). A negative value suggests high ammonium excretion, as with diarrhea; a positive value in metabolic acidosis suggests a renal cause such as distal renal tubular acidosis.
Source: Anion Gap and Non-Anion Gap Metabolic Acidosis (NCBI Bookshelf, NIH) [1]
Sources
Every formula, cutoff and reference value on this page comes from the official sources below. Click any source to check it yourself.
- Anion Gap and Non-Anion Gap Metabolic Acidosis. StatPearls (updated 6 August 2025) (NCBI Bookshelf, NIH)
- Biochemistry, Anion Gap. StatPearls (NCBI Bookshelf, NIH)
- Partial Pressure of Carbon Dioxide. StatPearls (updated 26 September 2022) (NCBI Bookshelf, NIH)
- USMLE Step 1 Sample Test Questions: USMLE Laboratory Values, pages 7 to 9 (PDF updated 2 April 2026, retrieved 24 October 2026) (USMLE program, usmle.org)
Last checked: 24 October 2026. Spotted a mistake? Please tell us on the Corrections page.
For exam study, not for clinical use. This tool is for learning how the formulas work. It is not medical advice. See the Medical disclaimer.
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