Henderson-Hasselbalch Calculator
Henderson-Hasselbalch Calculator
Calculate pH from bicarbonate and pCO₂ using the equation blood gas analysers themselves rely on.
Henderson-Hasselbalch
pH from HCO₃⁻/pCO₂HCO₃⁻ 24 mmol/L, pCO₂ 40 mmHg
Formula
- 6.1
- the pKa of the carbonic acid / bicarbonate buffer system at 37°C
- 0.03
- the solubility coefficient of CO₂ in plasma, mmol/L per mmHg — converts pCO₂ into dissolved carbonic acid
- HCO₃⁻
- bicarbonate, mmol/L
Worked example
HCO₃⁻ 24 mmol/L, pCO₂ 40 mmHg
0.03 × 40 = 1.2
24 ÷ 1.2 = 20
6.1 + log₁₀(20) = 6.1 + 1.30 = 7.40
The Henderson variant
| Form | Equation |
|---|---|
| Henderson-Hasselbalch (pH) | pH = 6.1 + log₁₀(HCO₃⁻ ÷ (0.03 × pCO₂)) |
| Henderson (linear, [H⁺]) | [H⁺] (nmol/L) ≈ 24 × pCO₂ ÷ HCO₃⁻ |
Where the equation comes from, and its circularity on a blood gas
The equation follows directly from the equilibrium between dissolved CO₂, carbonic acid and bicarbonate. Its pKa of 6.1 is the effective, apparent pKa of this open buffer system at 37°C — lower than carbonic acid’s true pKa because CO₂ is continuously exhaled, which is what makes bicarbonate such an effective buffer despite starting far from its nominal pKa. The 0.03 term converts pCO₂ in mmHg into the concentration of dissolved CO₂ in mmol/L, the solubility coefficient of CO₂ in plasma.
A detail worth knowing changes how a discrepancy should be read. Blood gas analysers measure pH and pCO₂ directly with electrodes, then calculate bicarbonate from this same equation rather than measuring it — so the bicarbonate on a gas report and the bicarbonate on a venous chemistry panel, which is measured directly by a different method, are not two independent readings of the same quantity. When they disagree materially, the usual explanation is that the two samples were not drawn at the same time, not that one method is wrong.
A useful bedside shortcut is the Henderson, linear form: hydrogen ion concentration in nmol/L is approximately 24 times pCO₂ divided by bicarbonate. It avoids logarithms entirely and converts back to pH only at the end, which makes rapid mental estimation at the bedside considerably easier than working with the logarithmic form directly.
Frequently asked questions
What does the 6.1 in the Henderson-Hasselbalch equation mean?
It is the apparent pKa of the bicarbonate buffer system in plasma at 37°C — lower than carbonic acid’s intrinsic pKa because CO₂ is continuously exhaled, keeping the open system buffering effectively.
Is the bicarbonate on a blood gas measured or calculated?
Calculated. Blood gas analysers measure pH and pCO₂ directly, then compute bicarbonate from this equation — it is not an independent measurement in the way a venous chemistry-panel bicarbonate is.
Why might blood gas and chemistry-panel bicarbonate disagree?
Because they come from different samples measured by different methods. A material discrepancy usually means the two samples were drawn at different times rather than that one result is wrong.
What is the Henderson equation, and how does it differ?
A linear rearrangement, [H⁺] in nmol/L ≈ 24 × pCO₂ ÷ HCO₃⁻, that avoids logarithms and is easier to estimate mentally at the bedside than the logarithmic Henderson-Hasselbalch form.
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References
- Kellum JA. Determinants of blood pH in health and disease. Crit Care. 2000;4(1):6–14.
- Adrogué HJ, Madias NE. Management of life-threatening acid-base disorders. N Engl J Med. 1998;338(1):26–34.