About the Arterial Blood pH Calculator
This calculator estimates arterial blood pH from two measured values — bicarbonate (HCO3-) and the arterial partial pressure of carbon dioxide (PaCO2) — using the Henderson-Hasselbalch equation. It is intended for educational and informational use. Medical calculations require clinical context that a calculator cannot provide, so always review outputs with a qualified clinician before making any medical decision.
The formula
Arterial pH is set by the bicarbonate buffer system, and the Henderson-Hasselbalch equation expresses it directly:
pH = 6.1 + log10( [HCO3-] / (0.03 x PaCO2) )
- 6.1 is the pKa of the carbonic acid / bicarbonate buffer pair near body temperature.
- [HCO3-] is plasma bicarbonate in mmol/L (equivalent to mEq/L) — the metabolic component.
- 0.03 mmol/L/mmHg is the solubility coefficient of CO2 in plasma, converting PaCO2 into dissolved CO2.
- PaCO2 is the arterial partial pressure of carbon dioxide in mmHg — the respiratory component.
With normal inputs (HCO3- = 24 mmol/L and PaCO2 = 40 mmHg), dissolved CO2 is 0.03 x 40 = 1.2 mmol/L, the bicarbonate-to-CO2 ratio is 24 / 1.2 = 20, and log10(20) = 1.30, so pH = 6.1 + 1.30 = 7.40 — the center of the normal range.
Interpreting the result
- Normal: arterial pH 7.35 to 7.45.
- Acidemia: pH below 7.35 — the blood is more acidic than normal.
- Alkalemia: pH above 7.45 — the blood is more alkaline than normal.
- The hydrogen ion [H+] value (in nmol/L) is the same information on a linear scale — about 40 nmol/L at pH 7.40, rising as pH falls and falling as pH rises.
- The 20:1 ratio of bicarbonate to dissolved CO2 is the hinge of the equation; anything that changes either component shifts the ratio, and therefore the pH.
A pH value alone does not identify the underlying disorder. Whether a change is respiratory (driven by PaCO2) or metabolic (driven by HCO3-), acute or chronic, and whether compensation is present all require the full arterial blood gas panel and clinical assessment.
Assumptions and limits
- The equation assumes a pKa of 6.1 and a CO2 solubility of 0.03, standard values for blood near 37 C; large temperature or protein shifts can change them slightly.
- Reference ranges vary by laboratory and patient population; your clinician's lab-specific ranges take precedence over any general guideline.
- This tool does not diagnose acid-base disorders, judge compensation, or account for the anion gap, medications, or overall clinical status.
Important disclaimer
This tool does not constitute medical advice. Do not use it to diagnose, treat, or manage any medical condition without consulting a licensed healthcare provider.