Winter's Formula Calculator

How it is calculated

The tool applies expected PaCO₂ = 1.5 × HCO₃⁻ + 8, where bicarbonate is the serum HCO₃⁻ in mEq/L (mmol/L) and the result is in mmHg. It then shows the usual tolerance of ±2, so you get an expected value plus a low and a high bound. The formula describes the respiratory compensation expected in a metabolic acidosis.

Only a bicarbonate above zero is accepted; zero or negative input gives no result. The calculator does not diagnose anything: it does not check pH, does not confirm that an acidosis is present, and uses PaCO₂ in mmHg only, so a value in kPa must be converted first. Compare the range with the patient's measured PaCO₂ from the same blood gas, and interpret it with the clinical picture.

Common mistakes

• Using it for a problem that is not a metabolic acidosis. The formula is built for that case; for a metabolic alkalosis or a primary respiratory disorder it does not apply.
• Entering the anion gap, base excess or total CO₂ instead of the serum bicarbonate (HCO₃⁻).
• Comparing the result with a PaCO₂ given in kPa. The output is in mmHg, so convert first.
• Forgetting the ±2 band and treating 26 as a single exact target. A measured value of 25 or 27 is still inside the expected range.
• Using the venous or capillary CO₂ as if it were arterial PaCO₂, which can differ from the arterial value.