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.
Tips
- Use values from the same blood sample when a formula combines several results.
- Check the units – sodium and potassium are usually mmol/L (mEq/L).
- Correct sodium slowly; rapid changes can cause brain injury.
- Re-check labs after any correction rather than relying on a single calculation.
Common mistakes to avoid
- Mixing mg/dL and mmol/L values.
- Using an IV rate without double-checking the drop factor of the giving set.
- Treating a calculated deficit as the exact amount to give.
Inputs explained
| Field | What to enter |
|---|---|
| Bicarbonate | In mEq/L |
| PaCO₂ | In mmHg |
| Measured 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 pH |
|---|---|
| 12 | 7.1 |
| 18 | 7.28 |
| 24 | 7.4 |
| 30 | 7.5 |
| 36 | 7.58 |
| 48 | 7.7 |
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.
Formula
pH = 6.1 + log₁₀(HCO₃⁻ ÷ (0.03 × PaCO₂))[H⁺] (nmol/L) = 10^(9 − pH)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.
Frequently asked questions
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.
What formula does the Blood pH Calculator use?
pH = 6.1 + log₁₀(HCO₃⁻ ÷ (0.03 × PaCO₂)).