🧪 Arterial Blood pH Calculator

Blood pH lives in a narrow corridor, 7.35 to 7.45, held there by the bicarbonate buffer system. The Henderson-Hasselbalch equation is the mathematical statement of that buffer: pH equals 6.1 plus the log of the bicarbonate-to-dissolved-CO2 ratio. Change either side and the pH moves predictably.

This calculator solves the equation from PaCO2 and bicarbonate and labels the result acidemic, normal, or alkalemic. It is the first step of every ABG interpretation and a favorite exam calculation.

How to use this calculator

Enter PaCO2 and bicarbonate from the arterial blood gas. The calculator derives pH and the underlying ratio.

The Henderson-Hasselbalch equation

The equation relates pH to the ratio of base (bicarbonate) to acid (dissolved CO2, which is 0.03 times PaCO2). The pK of 6.1 is the carbonic acid dissociation constant.

The 20-to-1 ratio is the number to memorize: normal blood holds about 20 parts bicarbonate to 1 part dissolved CO2, and the log of 20 is 1.3, giving 7.40. Halve the ratio and pH drops 0.3; double it and pH rises 0.3.

Worked example

PaCO2 30, bicarbonate 15. Dissolved CO2 is 0.03 x 30 = 0.9. The ratio is 15/0.9 = 16.7. Log10 of 16.7 is 1.22. pH = 6.1 + 1.22 = 7.32: acidemia. Bicarbonate is low, so this is a metabolic acidosis pattern (the DKA-style numbers from our acid-base examples).

Now PaCO2 60, bicarbonate 26. Dissolved CO2 is 1.8, ratio 14.4, log 1.16, pH 7.26: acidemia driven by high CO2, a respiratory acidosis pattern. Same acidemia, opposite cause, which is why the ratio matters more than the pH alone.

From pH to disorder

pH only says which direction; the components say why. Acidemia (under 7.35) with low bicarbonate is metabolic acidosis; with high PaCO2 it is respiratory acidosis. Alkalemia (over 7.45) with high bicarbonate is metabolic alkalosis; with low PaCO2 it is respiratory alkalosis.

The calculated pH should match the measured pH on the ABG within about 0.02. A bigger gap suggests a lab error or a transcription mistake, and rechecking beats rationalizing. For the complete workup including compensation and mixed disorders, see our ABG acid-base interpreter and anion gap calculator.

Why the body guards pH so tightly

Enzymes are pH-sensitive machines: shift blood pH by 0.3 and protein function degrades noticeably; shift it by 0.5 and life is threatened. The bicarbonate buffer handles minute-to-minute changes, the lungs adjust CO2 within minutes, and the kidneys rebuild bicarbonate over days.

This three-layer defense is why single disorders rarely kill directly and why mixed disorders are so dangerous: they can overwhelm layers simultaneously. The equation you just used is the quantitative face of the first and fastest layer.

The pH-first rule of ABG reading

Every ABG interpretation starts the same way: look at pH first. If it is low, you are hunting an acidosis; if high, an alkalosis. Then find which component matches: low bicarbonate with acidemia means metabolic acidosis, high PaCO2 with acidemia means respiratory acidosis. The matching component is primary; the other is compensation or a second disorder.

This order prevents the classic beginner error of calling the biggest number the primary disorder. A PaCO2 of 60 looks dramatic, but if the pH is 7.48 with bicarbonate 40, the primary disorder is metabolic alkalosis and the CO2 is compensation. pH first, always.

Base excess: the metabolic number

Base excess (or base deficit when negative) quantifies the metabolic component in a single number: the amount of acid or base needed to return the blood to pH 7.40 at normal CO2. Normal is -2 to +2. A base excess of -8 means a significant metabolic acidosis; +8 means significant metabolic alkalosis.

It is the number ICU clinicians watch during resuscitation: a base deficit that narrows as fluids and perfusion improve signals the metabolic acidosis resolving. Like pH itself, it summarizes but does not diagnose; the anion gap and clinical story still name the cause.

Venous vs arterial gases: what changes

Not every pH needs an arterial stab. Venous blood gases are easier, less painful, and adequate for many purposes, but the numbers shift: venous pH runs about 0.03 to 0.05 lower, venous PaCO2 about 5 to 8 mmHg higher, and venous bicarbonate is nearly identical to arterial. For metabolic disorders, a venous gas plus the clinical picture often suffices.

Where venous gases fail is oxygenation and precise respiratory assessment: venous PaO2 reflects tissue extraction, not lung function, and cannot substitute. The practical rule: venous for "is there a metabolic acidosis and how bad," arterial for "how are the lungs doing and what is the exact PaCO2."

Either way, the Henderson-Hasselbalch math is the same; only the reference expectations shift. Interpret venous numbers against venous norms, and convert mentally before applying arterial rules like Winter's formula.

For informational purposes only, not medical advice. Consult a healthcare professional.