Acid-Base Calculator — Blood Gas pH (Henderson-Hasselbalch)
Enter bicarbonate (HCO₃⁻, mmol/L) and arterial carbon dioxide pressure (PaCO₂, mmHg) to compute arterial pH via the Henderson-Hasselbalch equation and classify any acid-base disorder as respiratory or metabolic acidosis or alkalosis.
mmol/L
mmHg
Normal — Within normal range
- 1
Dissolved CO₂
0.0307 × 40 = 1.228Converts PaCO₂ (mmHg) to dissolved carbonic acid concentration in mmol/L. - 2
HCO₃⁻ ÷ dissolved CO₂
24 ÷ 1.228 = 19.544 - 3
Arterial pH
6.1 + log₁₀(19.544) = 7.39
How does this calculator work?
Arterial pH = 6.1 + log₁₀([HCO₃⁻] / (0.0307 × PaCO₂)). Normal: pH 7.35–7.45, HCO₃⁻ 22–26 mmol/L, PaCO₂ 35–45 mmHg. Low pH = acidosis (high PaCO₂ → respiratory; low HCO₃⁻ → metabolic). High pH = alkalosis (low PaCO₂ → respiratory; high HCO₃⁻ → metabolic). Clinical use requires full context.
Formula
How this is calculated
Blood pH is governed by the carbonate buffer system. Carbon dioxide dissolves in plasma to form carbonic acid (CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻), and the equilibrium between dissolved CO₂ and bicarbonate determines pH. The Henderson-Hasselbalch equation expresses this: pH = pKa + log([HCO₃⁻] / [H₂CO₃]). For blood, the effective pKa of the system is 6.1, and the dissolved-CO₂ concentration equals 0.0307 × PaCO₂ (in mmol/L per mmHg at body temperature). Together these give the widely used clinical formula above.
Normal arterial values are pH 7.35–7.45, HCO₃⁻ 22–26 mmol/L, and PaCO₂ 35–45 mmHg. A pH below 7.35 is acidaemia; above 7.45 is alkalaemia. Primary CO₂ elevation (hypoventilation, COPD, sedation) drives respiratory acidosis; primary CO₂ reduction (hyperventilation, anxiety, altitude) drives respiratory alkalosis. Primary HCO₃⁻ reduction (diabetic ketoacidosis, lactic acidosis, renal tubular acidosis) drives metabolic acidosis; primary HCO₃⁻ elevation (vomiting, diuretics, excess antacids) drives metabolic alkalosis. Mixed disorders occur when both components are abnormal simultaneously.
This tool provides a rapid reference calculation only. Full blood gas interpretation requires compensation status (expected compensatory response vs. measured values), anion gap, base excess, oxygenation parameters, and the patient’s full clinical picture. Results must be reviewed by a qualified clinician before any treatment decision.
Frequently asked questions
Normal arterial pH is 7.35–7.45. Below 7.35 is acidaemia (too much acid or too little base); above 7.45 is alkalaemia. pH below about 6.8 or above 7.8 is generally incompatible with life.
Respiratory disorders are driven by changes in PaCO₂ (controlled by breathing). Metabolic disorders are driven by changes in HCO₃⁻ (controlled by the kidneys and buffering systems). Each primary disorder triggers a partial compensatory response from the other system to restore pH toward normal.
No. It computes pH from entered values using a standard clinical equation, but diagnosis requires additional measurements (base excess, lactate, anion gap, SpO₂, electrolytes) and full clinical context. Always consult a qualified healthcare professional for clinical decisions.
Also known as
TG we-Calculate Editorial Team. (2026). Acid-Base Calculator — Blood Gas pH (Henderson-Hasselbalch) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/acid-base-calculator
TG we-Calculate Editorial Team. "Acid-Base Calculator — Blood Gas pH (Henderson-Hasselbalch)." TG we-Calculate. 2026. https://we-calculate.com/calculator/acid-base-calculator.
TG we-Calculate Editorial Team, "Acid-Base Calculator — Blood Gas pH (Henderson-Hasselbalch)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/acid-base-calculator
@misc{wecalculate_acid_base_calculator, title = {Acid-Base Calculator — Blood Gas pH (Henderson-Hasselbalch)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/acid-base-calculator}}, year = {2026}, note = {TG we-Calculate} }
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