Venous Blood pH Calculator — Henderson-Hasselbalch Venous Gas
Enter venous bicarbonate (HCO₃⁻, mmol/L) and venous carbon dioxide pressure (PvCO₂, mmHg) to calculate venous blood pH via the Henderson-Hasselbalch equation and estimate the corresponding arterial pH.
mmol/L
mmHg
Normal venous pH
- 1
Dissolved CO₂ (α × PvCO₂)
0.0307 × 45 = 1.3815The CO₂ solubility coefficient α = 0.0307 mmol/L per mmHg. - 2
Bicarbonate-to-dissolved ratio
24 ÷ 1.3815 = 17.372 - 3
Venous pH (Henderson-Hasselbalch)
6.1 + log₁₀(17.372) = 7.34
How does this calculator work?
Venous pH = 6.1 + log₁₀([HCO₃⁻] / (0.0307 × PvCO₂)). Normal venous: pH 7.31–7.41, HCO₃⁻ 22–26 mmol/L, PvCO₂ 41–51 mmHg. Arterial pH ≈ venous pH + 0.04 in stable patients (gap widens in shock). For clinical decisions, always review with a qualified clinician.
Formula
How this is calculated
Venous blood gas (VBG) measurement is increasingly used as a less invasive surrogate for arterial blood gas (ABG) in certain clinical settings, as it avoids the discomfort and risk of arterial puncture. The chemistry is identical: venous pH is calculated from the bicarbonate concentration and partial pressure of CO₂ using the Henderson-Hasselbalch equation with pKa = 6.1 and a CO₂ solubility coefficient of 0.0307 mmol/L per mmHg.
Venous blood has different normal ranges from arterial blood because tissue metabolism adds CO₂ and consumes oxygen between the arterial and venous sides of the circulation. Normal venous pH is 7.31–7.41 (versus arterial 7.35–7.45), PvCO₂ is 41–51 mmHg (versus arterial 35–45 mmHg), and HCO₃⁻ is similar (22–26 mmol/L). The venous-to-arterial pH gradient is typically 0.02–0.05 pH units, but this gap widens substantially in low-cardiac-output states such as heart failure or shock — making venous pH a less reliable arterial surrogate in those situations.
This calculator computes venous pH and provides a rough estimated arterial pH by adding 0.04. This estimate is only valid in haemodynamically stable patients with normal perfusion. Full clinical interpretation requires base excess, anion gap, lactate, and the patient's clinical context. Results must be reviewed by a qualified clinician.
Frequently asked questions
Venous pH is typically 0.02–0.05 units lower than arterial pH because tissue metabolism generates CO₂ that acidifies venous blood. Normal arterial pH is 7.35–7.45; normal venous pH is 7.31–7.41. The gap widens in circulatory shock or heart failure, making venous-to-arterial correlation unreliable in those states.
VBG is used when continuous or repeated blood gas monitoring is needed (arterial puncture is technically more difficult and riskier), in haemodynamically stable patients where the extra arterial information is unlikely to change management, and for checking acid-base or HCO₃⁻ when oxygenation is separately assessed by pulse oximetry. It is not adequate for evaluating hypoxaemia or Pa O₂.
No. It computes pH from the Henderson-Hasselbalch equation using the entered values, but clinical interpretation requires additional parameters (base excess, anion gap, lactate, SpO₂, electrolytes, clinical signs) and must be performed by a qualified healthcare professional. Never rely on this tool alone for any treatment decision.
Also known as
TG we-Calculate Editorial Team. (2026). Venous Blood pH Calculator — Henderson-Hasselbalch Venous Gas [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/venous-blood-ph-calculator
TG we-Calculate Editorial Team. "Venous Blood pH Calculator — Henderson-Hasselbalch Venous Gas." TG we-Calculate. 2026. https://we-calculate.com/calculator/venous-blood-ph-calculator.
TG we-Calculate Editorial Team, "Venous Blood pH Calculator — Henderson-Hasselbalch Venous Gas," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/venous-blood-ph-calculator
@misc{wecalculate_venous_blood_ph_calculator, title = {Venous Blood pH Calculator — Henderson-Hasselbalch Venous Gas}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/venous-blood-ph-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?