Enter the pH and PaCO₂ (and bicarbonate if reported) to get HCO₃⁻ by Henderson–Hasselbalch, a consistency check, and the expected compensation for each acid–base process, including Winters' formula. Add sodium and chloride for the anion gap. For healthcare professionals.
mmol/L
Leave empty to calculate it from pH and PaCO₂
Anion gap (sodium, chloride, albumin)
mmol/L
mmol/L
HCO₃⁻ (Henderson–Hasselbalch)
10.1 mmol/L
[H⁺] from pH
60.3 nmol/L
pH, PaCO₂ and HCO₃⁻ fit together?
Yes (within 10%)
pH 7.22 is below the usual 7.35–7.45; PaCO₂ 25 mmHg is below 35–45 mmHg; HCO₃⁻ 10 is below 22–26 mmol/L (adult arterial reference intervals; check your laboratory's).
Henderson check: 24 × 25 ÷ 10 = 60 nmol/L against 60.3 from the pH.
Expected compensation for each possible primary process
If the primary process is…
Expected
Rule
Metabolic acidosis
PaCO₂ 21–25 mmHg
1.5 × HCO₃⁻ + 8 ± 2 (Winters)
Metabolic alkalosis
PaCO₂ 28.2–32.2 mmHg
40 + 0.7 × (HCO₃⁻ − 24) ± 2
Respiratory alkalosis
HCO₃⁻ 21 (acute) or 18 (chronic)
−2 or −4 per 10 mmHg PaCO₂ below 40
Measured PaCO₂ 25 mmHg, HCO₃⁻ 10 mmol/L. A value outside the expected range suggests a second process; which one is primary is a clinical judgement.
For healthcare professionals. Formulas and expected values, not an interpretation or a diagnosis: verify against current guidelines and the clinical picture. Reference intervals for children and animals differ.
How to use the blood gas calculator
Enter the pH and PaCO₂ in mmHg or kPa. Add the reported bicarbonate, or leave it empty to calculate it.
Open the anion gap section and add sodium, chloride and, if you have it, albumin.
Compare the measured values with the expected compensation for each possible primary process in the table.
Worked example
pH 7.22, PaCO₂ 25 mmHg and HCO₃⁻ 10 mmol/L. From the pH, [H⁺] = 10^(9 − 7.22) = 60.3 nmol/L; the Henderson check gives 24 × 25 ÷ 10 = 60, so the values fit together. Henderson–Hasselbalch gives HCO₃⁻ = 0.0307 × 25 × 10^1.12 = 10.1 mmol/L. If the primary process is a metabolic acidosis, Winters' formula predicts PaCO₂ = 1.5 × 10 + 8 = 23 ± 2, so 21–25 mmHg; the measured 25 mmHg is within it. With sodium 138 and chloride 100 the anion gap is 138 − 110 = 28 mmol/L.
Respiratory acidosis: HCO₃⁻ + 1 (acute) or + 3.5 (chronic) per 10 mmHg above 40
Respiratory alkalosis: HCO₃⁻ − 2 (acute) or − 4 (chronic) per 10 mmHg below 40
Source for the compensation rules: Berend K, de Vries APJ, Gans ROB, N Engl J Med 2014;371:1434. 1 kPa = 7.50062 mmHg. For healthcare professionals: expected values, not a diagnosis.
Frequently asked questions
How is bicarbonate calculated from a blood gas?
From the Henderson–Hasselbalch equation: HCO₃⁻ = 0.0307 × PaCO₂ (mmHg) × 10^(pH − 6.1). At pH 7.40 and PaCO₂ 40 mmHg that is about 24.5 mmol/L. Blood gas analysers use slightly different constants, so their calculated bicarbonate can differ by 1–2%.
What is Winters' formula?
The expected PaCO₂ when the body compensates for a metabolic acidosis: 1.5 × HCO₃⁻ + 8, plus or minus 2 mmHg. With bicarbonate 10 mmol/L the expected PaCO₂ is 21–25 mmHg. A measured PaCO₂ above the range suggests an added respiratory acidosis, and one below it an added respiratory alkalosis.
What are the rules for respiratory compensation?
For a respiratory acidosis, bicarbonate rises about 1 mmol/L for every 10 mmHg PaCO₂ above 40 when acute, and about 3.5 when chronic (some texts say 4). For a respiratory alkalosis it falls about 2 per 10 mmHg when acute and 4 when chronic (some say 5). The calculator shows both acute and chronic values for the PaCO₂ you enter.
Why check that pH, PaCO₂ and HCO₃⁻ fit together?
If the three numbers come from different samples, or one was mistyped, they won't satisfy the Henderson equation: [H⁺] = 24 × PaCO₂ ÷ HCO₃⁻, in nmol/L. The calculator compares that with the [H⁺] from the pH and flags a difference of more than 10%.
Can I use it for animals?
The formulas are the same, but the reference intervals shown are for adult humans, and expected compensation in dogs and cats differs. Use species-specific references from a veterinary laboratory or text.
