How this calculator works
Winters' approximation estimates respiratory compensation when metabolic acidosis is the primary disorder. A pressure outside the expected band suggests another respiratory process or an evolving response, requiring clinical interpretation. The inverse mode rearranges the midpoint only; it does not estimate measured bicarbonate from a blood gas independently. The result is an estimate for information only and is not medical advice.
Formula
Expected arterial PCO2 = 1.5 × HCO3 + 8 ± 2 mmHg. Midpoint inverse: HCO3 = (PCO2 - 8) / 1.5.
Worked example
Bicarbonate 12 mmol/L gives a midpoint of 26 mmHg and a range of 24 to 28. Bicarbonate 16 gives 32 mmHg, with a range of 30 to 34.
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Frequently asked questions
Are the limits inclusive?
Yes. A measured pressure equal to either endpoint is reported within the expected band.
Can this interpret venous CO2?
The approximation is for arterial PCO2. Venous and arterial CO2 pressures are not interchangeable.
Why reject bicarbonate of 24 or higher?
The page is scoped to primary metabolic acidosis. Other disturbances need different compensation rules.