Anion Gap Calculator
Compute the serum anion gap from your sodium, chloride and bicarbonate, with an optional potassium term. Add an albumin correction so a low albumin cannot hide a true acidosis, then use the delta gap and delta ratio to uncover a mixed acid-base disorder behind the main one.
Formula
Worked example
With Na⁺ = 140, Cl⁻ = 104 and HCO₃⁻ = 24 mEq/L, the gap is 140 − (104 + 24) = 12 mEq/L, the top of the normal range. If albumin is only 2.0 g/dL, the corrected gap is 12 + 2.5 × (4.0 − 2.0) = 17 mEq/L, which is genuinely elevated. Were the gap 24 and bicarbonate 14, the delta ratio is (24 − 12) / (24 − 14) = 1.2, a pure high-gap acidosis.
What the anion gap measures
The anion gap estimates the difference between routinely measured cations and anions in serum. It is calculated as sodium (optionally plus potassium) minus the sum of chloride and bicarbonate. Because the body stays electrically neutral, this "gap" reflects anions the standard panel does not measure directly: proteins (mainly albumin), phosphate, sulfate, and organic acids. A conventional normal range is 8-12 mEq/L without potassium, or roughly 12-16 mEq/L when potassium is added, although ranges differ between laboratories and many modern analyzers report lower values. Clinicians use the gap mainly to detect and classify metabolic acidosis.
Including potassium and switching units
Some laboratories add potassium to the cation side, which raises the gap and its normal range by about 4 mEq/L. Turn on the potassium option to match a panel that includes it; the calculator shifts the normal band automatically so the interpretation stays correct. Electrolyte values reported in mEq/L and mmol/L are numerically identical for these monovalent ions, so no conversion is needed for sodium, chloride, bicarbonate or potassium. Albumin is the exception: it is reported as g/dL in the United States and g/L elsewhere, so the albumin field lets you pick either unit and converts internally.
The albumin correction
Albumin is the largest contributor to the normal anion gap, so a low serum albumin can hide a clinically important high-gap acidosis. The standard adjustment adds roughly 2.5 mEq/L to the gap for every 1 g/dL that albumin falls below 4.0 g/dL (equivalently 0.25 mEq/L per 1 g/L below 40 g/L). In critically ill or malnourished patients with low albumin, using the corrected gap prevents a true acidosis from being overlooked. Toggle the albumin correction on and enter the measured albumin to see both the raw and corrected values side by side.
Delta gap and delta ratio for mixed disorders
Once a high-anion-gap metabolic acidosis is found, the delta ratio (also called the delta-delta) checks whether a second acid-base disorder is hiding behind it. It divides the rise in the anion gap above its baseline by the fall in bicarbonate below normal. In a pure high-gap acidosis each unit of acid consumes about one unit of bicarbonate, so the ratio sits near 1 to 2. A ratio below 0.4 points to a coexisting non-gap (hyperchloremic) acidosis, 0.4 to 0.8 suggests a mixed picture, and a ratio above 2 signals a concurrent metabolic alkalosis or a chronic respiratory acidosis that has retained bicarbonate. You can set your own baseline gap and bicarbonate if your laboratory or patient differs from the classic 12 and 24.
Important limits
This calculator provides general educational estimates and is not a substitute for medical evaluation. Reference ranges vary by laboratory and analyzer, and the anion gap must always be read together with the full metabolic panel, blood gas, glucose, lactate, and the clinical picture. The delta ratio is a guide, not a verdict, and performs best when the gap is clearly raised. An abnormal gap, or a normal gap in a symptomatic person, should be reviewed promptly by a qualified clinician, who will decide on any further testing or treatment. Never use this tool to self-diagnose or to delay seeking care.
Delta ratio (delta-delta) interpretation
| Delta ratio | Interpretation | Status |
|---|---|---|
| Below 0.4 | Pure non-gap (hyperchloremic) metabolic acidosis | Low |
| 0.4 to 0.8 | Mixed high-gap and non-gap metabolic acidosis | Low |
| 1.0 to 2.0 | Pure high-anion-gap metabolic acidosis | Normal |
| Above 2.0 | High-gap acidosis with a metabolic alkalosis or chronic respiratory acidosis | High |
The delta ratio compares the rise in the anion gap with the fall in bicarbonate. It helps reveal a second acid-base disorder hiding behind a high-gap acidosis. Read it together with the gap, not on its own.
Frequently asked questions
What is a normal anion gap?
A commonly cited normal range is 8-12 mEq/L when potassium is not included, or about 12-16 mEq/L when it is. Ranges depend on the laboratory and the method used to measure electrolytes, and many contemporary analyzers report lower normal gaps. Always interpret your result against your own lab’s reference range and the rest of your blood work.
Should potassium be included in the anion gap?
Both versions are valid. Most clinicians use the simpler sodium-only formula, but some laboratories add potassium, which raises the gap and its normal range by roughly 4 mEq/L. The key is consistency: use the same formula and the matching reference range each time. This calculator shifts the normal band automatically when you turn potassium on.
When should I correct the anion gap for albumin?
Correct when serum albumin is low, which is common in critically ill, hospitalized, or malnourished patients. Because albumin carries a negative charge, a low level lowers the measured gap and can mask a real high-gap acidosis. Add about 2.5 mEq/L for each 1 g/dL that albumin is below 4.0 g/dL (or 0.25 mEq/L per g/L below 40 g/L).
What does the delta ratio tell me?
The delta ratio compares how far the anion gap has risen with how far bicarbonate has fallen. A value near 1 to 2 fits a pure high-gap acidosis. Below about 0.8 suggests an additional non-gap acidosis, and above 2 suggests a coexisting metabolic alkalosis or a chronic respiratory acidosis. It is most useful once a high gap is confirmed and should be read alongside the blood gas.
Does a high anion gap always mean acidosis?
Not always, but a raised gap most often reflects a high-anion-gap metabolic acidosis from unmeasured acids such as lactate, ketones, or toxins. It can occasionally rise from other causes, including dehydration or laboratory artifact. A clinician interprets it alongside a blood gas and the full clinical context.