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Winters' Formula (Expected pCO₂ in Metabolic Acidosis)

Winters' formula predicts the pCO₂ in a compensated metabolic acidosis: expected pCO₂ = 1.5 × HCO₃ + 8 ± 2 mmHg. A measured pCO₂ above the range means a concurrent respiratory acidosis; below it, a concurrent respiratory alkalosis.

  • Serum bicarbonate (HCO₃⁻) (mmol/L or mEq/L)
  • Measured pCO₂ (optional) (mmHg or kPa)
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Interpretation

ResultMeaning
Measured > expected + 2Additional respiratory acidosis
Within ± 2Appropriate compensation
Measured < expected − 2Additional respiratory alkalosis

Formula

Expected pCO₂ (mmHg) = 1.5 × HCO₃⁻ + 8 ± 2. (kPa = mmHg ÷ 7.5)

Clinical pearls

  • Only valid for a primary metabolic acidosis; respiratory compensation takes 12–24 h to be complete.
  • pCO₂ rarely falls below 10–12 mmHg even with maximal compensation.

Examples

winters formula hco3 12 pco2 35expected pco2 bicarb 8is respiratory compensation appropriate: bicarbonate 15, PaCO2 4.0 kPa

FAQ

What is Winters' formula?

Expected pCO₂ = 1.5 × bicarbonate + 8 (± 2) mmHg, the respiratory compensation expected in metabolic acidosis.

What if the pCO₂ is higher than predicted?

There is a concurrent respiratory acidosis (inadequate ventilation), e.g. from fatigue, sedation or neuromuscular weakness.

Evidence

  1. Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med 1967;66:312–22.
  2. Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med 2014;371:1434–45.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology