How this instrument works
Blood pH is not measured by intuition; it is set by a chemical balance between carbon dioxide dissolved in plasma and the bicarbonate buffering it. The Henderson-Hasselbalch equation is the mathematical expression of that balance: pH equals 6.1 (the effective pKa of the carbonic acid system in blood) plus the base-10 logarithm of bicarbonate divided by 0.03 times PaCO₂, where 0.03 is the solubility coefficient converting the CO₂ partial pressure into a dissolved-gas concentration comparable to bicarbonate. Feed it the same two figures a blood gas machine reports and it returns the same pH the machine would.
This is not a shortcut or an approximation layered on top of blood gas interpretation — it is the physiological foundation underneath it. Every bedside rule of thumb for reading an arterial blood gas, every compensation formula, every acid-base nomogram traces back to this single equation, described in standard respiratory and renal physiology references such as StatPearls' overview of acid-base balance. Clinical blood gas analyzers do not use a lookup table; they compute pH from measured PaCO₂ and derived bicarbonate through exactly this relationship.
What the equation cannot tell you is why bicarbonate or PaCO₂ landed where they did. A pH of 7.20 looks identical to this calculator whether it comes from a kidney that stopped generating bicarbonate or a lung that stopped clearing carbon dioxide — the number is the same, the story behind it is not. Distinguishing a metabolic disturbance from a respiratory one, and checking whether the body has partially compensated, requires looking at bicarbonate and PaCO₂ together rather than folding them into a single pH figure, which is the job of a separate acid-base classification step.
- Enter Bicarbonate (mEq/L) — the HCO₃⁻ value reported on the blood gas panel or basic metabolic panel.
- Enter PaCO₂ (mmHg) — the partial pressure of carbon dioxide from the same arterial sample.
- Read Arterial pH, computed to four decimal places from the two values above.
- Compare the result against the normal reference range of 7.35 to 7.45 to see which side, if either, it falls on.
- Re-run with a single value changed at a time to see how much a bicarbonate drop versus a PaCO₂ rise each move the pH on its own.
Worked example — three bicarbonate/PaCO₂ pairs
Start with bicarbonate at 24 mEq/L and PaCO₂ at 40 mmHg, both squarely normal. Divide: 24 ÷ (0.03 × 40) = 24 ÷ 1.2 = 20. Take the base-10 logarithm of 20, which is about 1.301, and add the constant 6.1: 6.1 + 1.301 = 7.401. That sits almost exactly in the middle of the normal 7.35-7.45 range, which is exactly what two normal inputs should produce.
Now drop bicarbonate to 15 mEq/L while leaving PaCO₂ at 40. The ratio becomes 15 ÷ 1.2 = 12.5, whose log₁₀ is about 1.097, giving 6.1 + 1.097 = 7.197, rounding to about 7.20 — an acidosis, and since PaCO₂ never moved, the falling bicarbonate is the entire reason the pH fell.
Instead, hold bicarbonate at 24 and raise PaCO₂ to 60 mmHg. Now 24 ÷ (0.03 × 60) = 24 ÷ 1.8 = 13.33, and log₁₀(13.33) is about 1.125, giving 6.1 + 1.125 = 7.225, rounding to about 7.22. That is a similar-looking acidosis to the bicarbonate case above, but it was produced entirely by carbon dioxide retention instead — a reminder that the same pH can arise from two very different underlying problems.
Questions
What counts as a normal arterial pH?
The accepted reference range is 7.35 to 7.45, with 7.40 treated as the physiological center. A value below 7.35 is acidemia and above 7.45 is alkalemia. Because the scale is logarithmic, small-looking shifts represent real changes in hydrogen ion concentration — a drop from 7.40 to 7.10 roughly doubles free hydrogen ions in the blood.
Why is the constant 6.1 instead of the textbook pKa of carbonic acid?
6.1 is not the pKa of pure carbonic acid in water; it is an effective, empirically fitted value for the carbonic acid-bicarbonate system as it behaves in whole blood plasma at body temperature, accounting for the equilibrium between dissolved CO₂ and true carbonic acid. It is the figure used throughout clinical physiology texts, including StatPearls' treatment of acid-base balance, and it is what makes the equation match measured blood gas values.
Can this calculator replace an arterial blood gas test?
No. It performs the arithmetic that connects bicarbonate and PaCO₂ to pH, but both of those inputs still have to come from a real blood sample analyzed in a lab or at the bedside. Treat this as a way to check the math or explore how sensitive pH is to each input, not as a substitute for drawing and running blood.
Does this tool identify the type of acid-base disturbance?
No — it only converts bicarbonate and PaCO₂ into a pH figure. Classifying whether a given pH-PaCO₂ combination points to a respiratory or metabolic disturbance, which is a distinct question, is handled by a separate classifier that takes measured pH and PaCO₂ as its own two inputs rather than computing pH from scratch.
Why does raising PaCO₂ lower pH?
Carbon dioxide dissolved in plasma reacts with water to form carbonic acid, so more CO₂ means more acid and a lower pH, all else equal. In the equation this shows up as PaCO₂ sitting in the denominator inside the logarithm: a bigger PaCO₂ shrinks the ratio, shrinks the logarithm, and pulls the whole sum down toward a lower pH.
What happens if bicarbonate or PaCO₂ is entered as zero?
The calculator blocks it. Zero bicarbonate is not physiologically real, and PaCO₂ has to be a positive pressure for the equation's logarithm and division to be defined at all — both inputs are checked and must be strictly greater than zero before a result is shown.
References
- StatPearls — Physiology, Acid Base Balance (NCBI Bookshelf)
- MedlinePlus — Arterial Blood Gas (ABG) Test
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.