Anion Gap Calculator
Free anion gap calculator with albumin correction. Enter sodium, chloride and bicarbonate to find the serum anion gap and see whether it is high, normal or low.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
Educational tool only. Reference ranges vary by lab and method. Not a substitute for clinical judgment or medical advice.
How the Anion Gap Calculator Works
The serum anion gap estimates the unmeasured anions in blood and helps evaluate metabolic acidosis. This calculator takes sodium, chloride, and bicarbonate (with optional potassium and albumin), then returns the anion gap, the albumin-corrected gap, and a basic interpretation.
The Formula
The standard anion gap is:
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
Some laboratories include potassium:
AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)
Because albumin is the dominant unmeasured anion, low albumin should be corrected for:
Corrected AG = AG + 2.5 × (4.0 − albumin in g/dL)
Worked Example
For Na⁺ = 140, Cl⁻ = 104, and HCO₃⁻ = 24 mEq/L with normal albumin = 4.0 g/dL:
- AG = 140 − (104 + 24)
- = 140 − 128 = 12 mEq/L
- Corrected AG = 12 + 2.5 × (4.0 − 4.0) = 12 mEq/L (no correction needed)
A gap of 12 sits at the upper edge of the typical 8–12 mEq/L range.
Reference Ranges
| Formula used | Typical normal range |
|---|---|
| Without potassium | 8–12 mEq/L |
| With potassium | 12–16 mEq/L |
| Albumin correction | +2.5 per 1 g/dL below 4.0 |
Important Notes
- Ranges vary by lab — always compare with your laboratory’s reference interval.
- Correct for low albumin to avoid missing a true high-gap acidosis.
- A normal gap does not rule out a mixed acid-base disorder.
- This tool is educational only and is not a substitute for clinical evaluation or medical advice.
Frequently asked questions
How do you calculate the anion gap?+
The anion gap is sodium minus the sum of chloride and bicarbonate: AG = Na⁺ − (Cl⁻ + HCO₃⁻), all in mEq/L. Some labs include potassium, giving AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). The gap represents unmeasured anions in the blood and helps classify metabolic acidosis.
What is a normal anion gap?+
When potassium is not included, a normal anion gap is roughly 8 to 12 mEq/L; when potassium is included it is about 12 to 16 mEq/L. Reference ranges vary between laboratories and methods, so always compare against your own lab's stated range rather than a fixed number.
Why correct the anion gap for albumin?+
Albumin is the main unmeasured anion, so low albumin lowers the measured gap and can mask a true high-gap acidosis. The correction adds about 2.5 mEq/L to the gap for every 1 g/dL that albumin falls below 4.0 g/dL: corrected AG = AG + 2.5 × (4.0 − albumin).
What causes a high anion gap?+
A high anion gap usually reflects added acids and is often remembered by mnemonics like MUDPILES or GOLD MARK: methanol, uremia, diabetic ketoacidosis, propylene glycol, isoniazid, lactic acidosis, ethylene glycol and salicylates. A clinician interprets the gap alongside the full clinical picture.
Is this anion gap calculator a diagnosis?+
No. This tool computes the anion gap from values you enter and flags whether it falls outside a typical reference range, but it does not diagnose any condition. Anion gap must be interpreted with the patient's history, blood gas and other labs by a qualified clinician. It is for education only.