For healthcare professionals
Anion Gap Calculator
The serum anion gap is sodium minus the sum of chloride and bicarbonate. It estimates unmeasured anions and is used in sorting out a metabolic acidosis. Because albumin is the main unmeasured anion, a low albumin lowers the gap, so this tool can also give the albumin-corrected anion gap. It is written for clinicians and is not a diagnosis.
Laboratory values
Use values from the same blood sample. Potassium and albumin are optional. Nothing you enter leaves this page.
What you will see
The anion gap with the working, the gap with potassium if you enter it, and the albumin-corrected gap if you enter albumin.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
The formulas as printed in two open-access (CC BY) studies: Chawla LS and colleagues, BMC Emergency Medicine 2008, and Zhong L and colleagues, Internal and Emergency Medicine 2022. Both correct for albumin with the Figge equation, using 4.4 g/dL as the reference albumin. The tests check each formula, the reference albumin and that a negative gap is shown as it is.
Validated in
Adults in intensive care. Chawla 2008 studied critically ill patients at one US hospital; Zhong 2022 studied 708 adults with acute kidney injury on continuous renal replacement therapy (the MIMIC-IV database and one hospital in China).
Limits
Normal ranges differ between laboratories and with whether potassium is included (Zhong 2022). A normal anion gap does not rule out a raised lactate (Chawla 2008). A low albumin lowers the measured gap and can hide a raised one (Chawla 2008). As a predictor of death in critical care, studies of the anion gap were too inconsistent to pool (Glasmacher 2016).
The formulas
- Anion gap = sodium - (chloride + bicarbonate) (Chawla 2008).
- Anion gap with potassium = (sodium + potassium) - (chloride + bicarbonate) (Zhong 2022).
- Albumin-corrected anion gap = anion gap + (4.4 - albumin in g/dL) x 2.5, the Figge equation (Chawla 2008) (Zhong 2022).
Worked example: sodium 140, chloride 104 and bicarbonate 24 mmol/L give an anion gap of 140 - (104 + 24) = 12 mmol/L. With an albumin of 2.4 g/dL, the corrected gap is 12 + (4.4 - 2.4) x 2.5 = 17 mmol/L.
Reading the result
Compare the result with your own laboratory's reference range. In one study the normal range for the anion gap with potassium was 8 to 20 mmol/L in a US intensive care database and 8 to 16 mmol/L at a hospital in China (Zhong 2022). Chawla and colleagues used a clinical threshold of 10 to 12 mEq/L for the gap without potassium (Chawla 2008). Zhong and colleagues grouped an albumin-corrected gap of 12 to 20 mmol/L as normal and over 20 mmol/L as high (Zhong 2022).
What the studies found
In critically ill patients, the anion gap and base deficit were poor tests for a raised lactate, and hypoalbuminemia, which is common in critical illness, can lower the measured gap and mask a raised one (Chawla 2008). In 708 adults starting continuous renal replacement therapy, a high albumin-corrected gap was linked with higher death in intensive care (Zhong 2022). A systematic review of the anion gap as a predictor of death in critical care found results too varied to pool, and suggested that intravenous fluids affect how well it predicts death (Glasmacher 2016).
Related tools
In a normal anion gap acidosis, the urine anion gap calculator helps separate gut from kidney causes. For acute kidney injury, the FENa calculator gives the fractional excretion of sodium. To check the respiratory response to a metabolic acidosis, see the Winters formula calculator. To correct sodium for high glucose, see the corrected sodium calculator. To grade organ failure in the intensive care unit, see the SOFA score calculator. To adjust total calcium for a low albumin, see the corrected calcium calculator. For suspected toxic alcohols, the serum osmolality calculator gives the osmolal gap. Other bedside scores are listed under For healthcare professionals. To compare the rise in the anion gap with the fall in bicarbonate, use the delta ratio calculator.
Sources
- Chawla LS, Shih S, Davison D, Junker C, Seneff MG. Anion gap, anion gap corrected for albumin, base deficit and unmeasured anions in critically ill patients: implications on the assessment of metabolic acidosis and the diagnosis of hyperlactatemia. BMC Emerg Med. 2008;8:18. doi:10.1186/1471-227X-8-18. PMID 19087326. Open access (CC BY). Methods (AG and Figge albumin-corrected AG formulas) and Discussion (clinical threshold 10 to 12 mEq/L; hypoalbuminemia masks a raised gap; poor test for hyperlactatemia). pmc.ncbi.nlm.nih.gov/articles/PMC2644323/
- Zhong L, Xie B, Ji XW, et al. The association between albumin corrected anion gap and ICU mortality in acute kidney injury patients requiring continuous renal replacement therapy. Intern Emerg Med. 2022;17(8):2315-2322. doi:10.1007/s11739-022-03093-8. PMID 36112320. Open access (CC BY). Methods (AG with potassium; ACAG formula; normal AG 8 to 20 and 8 to 16 mmol/L; ACAG groups 12 to 20 and over 20 mmol/L) and Results (708 patients). pmc.ncbi.nlm.nih.gov/articles/PMC9652260/
- Glasmacher SA, Stones W. Anion gap as a prognostic tool for risk stratification in critically ill patients: a systematic review and meta-analysis. BMC Anesthesiol. 2016;16:68. doi:10.1186/s12871-016-0241-y. PMID 27577038. Open access (CC BY). Results (high heterogeneity; pooled estimates not to be interpreted; intravenous fluids). pmc.ncbi.nlm.nih.gov/articles/PMC5006450/
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.