Calculating your anion gap is straightforward once you know the Anion Gap formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Anion Gap Calculator.
What is Anion Gap?
The Anion Gap calculation tells you your anion gap from a few simple inputs. The figure you are solving for here is the anion gap.
The Anion Gap formula
The core formula is:
Anion gap = Sodium (Na⁺) - (Chloride (Cl⁻) + Bicarbonate (HCO₃⁻))Here is what each input means:
- Sodium (Na⁺) — a value measured in mEq/L. Example: 140 mEq/L.
- Chloride (Cl⁻) — a value measured in mEq/L. Example: 104 mEq/L.
- Bicarbonate (HCO₃⁻) — a value measured in mEq/L. Example: 24 mEq/L.
How to calculate it step by step
- Write down the sodium (na⁺) (for example, 140 mEq/L).
- Write down the chloride (cl⁻) (for example, 104 mEq/L).
- Write down the bicarbonate (hco₃⁻) (for example, 24 mEq/L).
- Apply the formula above to get your anion gap.
- Double-check the result with the Anion Gap Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Sodium (Na⁺) | 140 mEq/L |
| Chloride (Cl⁻) | 104 mEq/L |
| Bicarbonate (HCO₃⁻) | 24 mEq/L |
| Anion gap | 12.0 |
With sodium (na⁺) of 140 mEq/L, chloride (cl⁻) of 104 mEq/L and bicarbonate (hco₃⁻) of 24 mEq/L, the anion gap works out to 12.0.
Example 2
With sodium (na⁺) of 280 mEq/L, chloride (cl⁻) of 104 mEq/L and bicarbonate (hco₃⁻) of 24 mEq/L, the anion gap works out to 152.0.
| Result | Value |
|---|---|
| Anion gap | 152.0 |
Example 3
With sodium (na⁺) of 70 mEq/L, chloride (cl⁻) of 104 mEq/L and bicarbonate (hco₃⁻) of 24 mEq/L, the anion gap works out to -58.0.
| Result | Value |
|---|---|
| Anion gap | -58.0 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
- These figures are general estimates, not medical advice — check with a qualified professional before acting on them.
Prefer not to do the maths by hand? — the Anion Gap Calculator does it instantly, for free, with the formula and a worked example built in.
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