# Henderson-Hasselbalch Equation (pH / HCO₃⁻ / PCO₂) Solve any one of pH/HCO₃⁻/PaCO₂ from the other two via pH = 6.1 + log10(HCO₃⁻/(0.03×PaCO₂)). Reports linear Henderson [H⁺] cross-check. Derived from Henderson 1908, Hasselbalch 1916, Siggaard-Andersen 1974, LITFL, StatPearls, Berend 2016. Not medical advice. > Canonical page: https://elysiatools.com/en/tools/bicarbonate-ph-conversion - **Category:** Health - **Keywords:** henderson hasselbalch, bicarbonate, ph, paco2, acid base, blood gas, abg, hydrogen ion, buffer, pka, nephrology, critical care ## Overview The Henderson-Hasselbalch calculator solves for pH, HCO₃⁻, or PaCO₂ when the other two values are provided. It also reports a linear Henderson [H⁺] cross-check and the calculated acid-base category. This tool is for equation-based reference and is not medical advice. ## Inputs - **pH (optional)** (number): Arterial pH. Provide any TWO of pH / HCO₃⁻ / PaCO₂; the tool solves the third. If all three are provided, the tool checks Henderson-Hasselbalch consistency instead. - **HCO₃⁻ (mmol/L, optional)** (number): Bicarbonate (mmol/L). Provide any TWO of pH / HCO₃⁻ / PaCO₂. - **PaCO₂ (mmHg, optional)** (number): Arterial PaCO₂ (mmHg). Provide any TWO of pH / HCO₃⁻ / PaCO₂. - **Decimal Places** (number) ## When to use - Calculate pH from bicarbonate and arterial PaCO₂ values. - Calculate HCO₃⁻ or PaCO₂ when pH and the other variable are known. - Check whether three entered values are consistent with the Henderson-Hasselbalch equation. ## How it works - Enter any two of pH, HCO₃⁻ in mmol/L, and PaCO₂ in mmHg. - The calculator applies pH = 6.1 + log10(HCO₃⁻ / (0.03 × PaCO₂)) or the rearranged equation for the missing variable. - The result includes the solved value, the input values, a pH category, and the calculation method. - A linear Henderson estimate of \[H⁺\] is reported as a cross-check; entering all three values checks equation consistency. ## Use cases - Verify textbook and classroom examples involving pH, bicarbonate, and PaCO₂. - Cross-check arterial blood gas equation calculations in acid-base study or review. - Compare measured values with the Henderson-Hasselbalch relationship during clinical education. ## Frequently asked questions ### What inputs does the calculator require? Provide any two of pH, HCO₃⁻, and PaCO₂. The tool solves for the third. ### What units should I use? Enter HCO₃⁻ in mmol/L and PaCO₂ in mmHg. Enter pH as a pH value such as 7.40. ### Can I enter all three values? Yes. When all three are entered, the calculator checks their Henderson-Hasselbalch consistency instead of solving for a missing value. ### What does the \[H⁺\] result mean? It is a linear Henderson cross-check calculated from [H⁺] = 24 × PaCO₂ / HCO₃⁻ and is reported in nmol/L. ### Is this a complete blood gas interpretation? No. The tool focuses on the Henderson-Hasselbalch equation and does not provide complete clinical interpretation or medical advice. ## Related tools - [A-a Gradient (Alveolar-Arterial O₂)](https://elysiatools.com/en/tools/a-a-gradient-calculator): Calculate the alveolar-arterial O₂ gradient to distinguish hypoxemia causes. PAO₂ = FiO₂×(P_atm−47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂. Normal ≈ (Age+10)/4. Elevated → V/Q mismatch, shunt, diffusion impairment. 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