Winter's Formula Calculator

About this calculator

Winter's formula, expected PaCO₂ = 1.5 × [HCO₃⁻] + 8 ± 2, predicts how much a patient's lungs should be blowing off CO₂ to compensate for a metabolic acidosis. It's derived from the empirical relationship between measured bicarbonate and the respiratory compensation seen in acid-base studies, giving an expected range rather than a single number.

Emergency and critical care physicians apply it when reading an arterial blood gas alongside a low bicarbonate: if the patient's actual PaCO₂ falls within the predicted range, the respiratory system is compensating appropriately for the metabolic acidosis alone; if it's higher or lower than expected, a second, independent respiratory acidosis or alkalosis is also present — a mixed disorder that changes management.