Equation Calculator

Calculates pH from bicarbonate and carbon dioxide using the Henderson–Hasselbalch relationship.

ABG Inputs

Typical adult reference values: PaCO₂ ≈ 35–45 mmHg and HCO₃⁻ ≈ 22–26 mEq/L.

Computed pH

Acid–Base Status
Henderson–Hasselbalch Formula:
Solving steps:
PaCO₂ mmHg
HCO₃⁻ mEq/L
Solving steps:
pH (Clinical Threshold View)
Reference:7.35–7.45
Acidemia Alkalemia
Ratio and denominator:

Interpretation Summary

Interpretive form:
Result:
Estimated hydrogen ion trend
Approximate normal pH: 7.40
Current pH: 7.40

Clinical Significance

Henderson Hasselbalch Equation

This calculator expands that idea by identifying the relevant measured values, substituting the selected inputs, and carrying the arithmetic through in the solving steps.

Clinically, the equation is useful for understanding why pH falls when carbon dioxide rises and why pH rises when bicarbonate increases. It also explains why the bicarbonate value reported on many arterial blood gas panels is calculated from measured pH and PaCO₂ rather than directly measured on the blood gas instrument. In bedside interpretation, the equation reinforces that acid–base disorders reflect a disturbance in the ratio between metabolic base and respiratory acid.

Despite its central role, the Henderson–Hasselbalch approach has limitations. It is a model of equilibrium and does not by itself identify the cause of an acid–base disorder, quantify compensation, or capture all contributors to nonbicarbonate buffering. Contemporary acid–base analysis may also incorporate anion gap, strong ion approaches, lactate, albumin, and clinical context. Accordingly, the equation should be integrated with the full ABG, serum chemistry, and the patient’s presentation rather than used in isolation.

References (APA 7th Edition)

  1. Castro, D., & Keenaghan, M. (2024). Arterial blood gas. StatPearls Publishing.
  2. Merck Manual Professional Edition. (2025). Acid-base regulation.
  3. Merck Manual Professional Edition. (2025). Acid-base disorders.
  4. OpenAnesthesia. (2026). Arterial blood gas analysis.
  5. OpenStax. (2019). Chemistry 2e: Buffers.
  6. OpenStax. (2022). Anatomy and Physiology 2e: Acid-base balance.
  7. Todi, S. (2023). Arterial blood gas analysis: A new look at the old formula. Indian Journal of Critical Care Medicine.
  8. American Thoracic Society. (n.d.). Interpretation of arterial blood gases (ABGs).