# Acid–Base Calculator (ABG Interpretation)

URL: https://speedycalculator.com/calculators/health/acid-base/
Last updated: 2026-10-30

> The acid–base calculator interprets an arterial blood gas: it identifies the primary disorder from pH, PaCO₂ and bicarbonate, checks whether compensation is appropriate with the standard formulas, and calculates the anion gap.

## Inputs

- pH
- PaCO₂ (mmHg)
- Bicarbonate (mEq/L)
- Sodium (mEq/L)
- Chloride (mEq/L)
- If respiratory: duration

## Results

- Primary disorder
- Expected compensation
- Compensation
- Anion gap

## Formula

```
Metabolic acidosis: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 (Winters)
Metabolic alkalosis: expected PaCO₂ = 0.7 × HCO₃⁻ + 21 ± 2
Respiratory acidosis: HCO₃⁻ rises 1 (acute) or 3.5 (chronic) per 10 mmHg PaCO₂
Respiratory alkalosis: HCO₃⁻ falls 2 (acute) or 5 (chronic) per 10 mmHg
```

## Explanation

Blood gas interpretation follows a sequence: is the pH acidaemic or alkalaemic, is the cause respiratory or metabolic, is compensation appropriate, and – in metabolic acidosis – what is the anion gap? Inappropriate compensation means a second, mixed disorder.

### How to use the Acid–Base Calculator

- **pH**
- **PaCO₂**
- **Bicarbonate**
- **Sodium**
- **Chloride**
- **If respiratory: duration**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter pH, PaCO₂ and bicarbonate, plus sodium and chloride for the anion gap, and say whether a respiratory problem is acute or chronic.

### Inputs explained

FieldWhat to enterpHA numberPaCO₂In mmHgBicarbonateIn mEq/LSodiumIn mEq/LChlorideIn mEq/LIf respiratory: durationChoose one: Acute, Chronic (more than 3–5 days)
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — pH: 7.3; PaCO₂: 30 mmHg; Bicarbonate: 15 mEq/L; Sodium: 140 mEq/L; Chloride: 105 mEq/L; If respiratory: duration: Acute.

Results — **Primary disorder = Metabolic acidosis**; Expected compensation = 30.5; Compensation = Appropriate; Anion gap = 20 mEq/L.

## Key terms

- **ABG**: Arterial blood gas.
- **Compensation**: The body's attempt to bring pH back toward normal.

## FAQ

**What are normal blood gas values?**
pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L.

**How do I tell metabolic from respiratory?**
If pH and bicarbonate move the same way, it is metabolic; if pH and PaCO₂ move opposite ways, it is respiratory.

**What is a mixed disorder?**
Two or more primary disorders at once, revealed by inappropriate compensation.


## Sources

- [Berend K et al. Physiological approach to assessment of acid–base disturbances. N Engl J Med, 2014](https://pubmed.ncbi.nlm.nih.gov/25295502/)