# Blood pH Calculator (Henderson–Hasselbalch)

URL: https://speedycalculator.com/calculators/health/arterial-blood-ph/
Last updated: 2026-10-30

> The blood pH calculator works out blood pH from bicarbonate and PaCO₂ with the Henderson–Hasselbalch equation, or the bicarbonate from pH and PaCO₂, and also gives the hydrogen ion concentration.

## Inputs

- Bicarbonate (mEq/L)
- PaCO₂ (mmHg)
- Measured pH (optional)

## Results

- Calculated pH
- Hydrogen ion concentration
- Measured minus calculated pH

## Formula

```
pH = 6.1 + log₁₀(HCO₃⁻ ÷ (0.03 × PaCO₂))
[H⁺] (nmol/L) = 10^(9 − pH)
```

## Explanation

The bicarbonate–carbon dioxide buffer is the body's main defence against changes in pH. Blood gas analysers measure pH and PaCO₂ and calculate bicarbonate with this equation, so it is useful for checking whether a set of results is internally consistent.

### How to use the Blood pH Calculator

- **Bicarbonate**
- **PaCO₂**
- **Measured pH (optional)**

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

### How it works

Enter bicarbonate and PaCO₂. If you also enter a measured pH, the calculator compares it with the calculated value.

### Inputs explained

FieldWhat to enterBicarbonateIn mEq/LPaCO₂In mmHgMeasured pH (optional)A number

### What each result means

- **Calculated pH** = 6.1 + log10([Bicarbonate]÷(0.03×[PaCO₂]))
- **Hydrogen ion concentration** = 10^(9 - [Calculated pH])

### Example values

How the main result changes when **Bicarbonate** changes and everything else stays at the example values above:

Bicarbonate (mEq/L)Calculated pH127.1187.28247.4307.5367.58487.7
*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 — Bicarbonate: 24 mEq/L; PaCO₂: 40 mmHg; Measured pH (optional): 0.

Results — **Calculated pH = 7.4**; Hydrogen ion concentration = 40 nmol/L; Measured minus calculated pH = 0.

## Key terms

- **pKa**: 6.1 for the bicarbonate buffer system.

## FAQ

**What is a normal blood pH?**
7.35–7.45.

**Why 0.03?**
It is the solubility of CO₂ in plasma in mmol/L per mmHg.

**What if measured and calculated pH differ?**
A difference over about 0.03 suggests a lab or sampling error.


## Sources

- [Hamm LL et al. Acid-base homeostasis. Clin J Am Soc Nephrol, 2015](https://pubmed.ncbi.nlm.nih.gov/26597304/)