Winter formula: expected PaCO2 in metabolic acidosis
Calculates the expected PaCO2 with its plus or minus 2 mmHg window and compares it with the measured value.
When to use
Metabolic acidosis where the adequacy of the respiratory response is the question.
Pearls and pitfalls
- The formula describes the respiratory compensation expected in metabolic acidosis: PaCO2 = 1.5 × bicarbonate + 8, with a window of plus or minus 2 mmHg.
- Measured PaCO2 is compared with the window, not only with the central value, so adequate compensation is not read as a mixed disorder.
- PaCO2 above the window indicates inadequate compensation or a concurrent respiratory acidosis; below it, a concurrent respiratory alkalosis.
- A published practical alternative is PaCO2 = 1.2 × bicarbonate + 11.2.
Fill in the fields to see the result.
Formula
Expected PaCO2 (mmHg) = 1.5 × bicarbonate (mmol/L) + 8, with a 2 mmHg window on each side.
Evidence
Winter formula applied step by step in blood gas interpretation, with the plus or minus 2 mmHg window and the reading of compensation.
References
- Muntingh J, Saffari S, Kelly A. Mastering blood gas interpretation: a practical guide for primary care providers. S Afr Fam Pract. 2025;67(1):6058. doi:10.4102/safp.v67i1.6058
- Turhan S, Tutan D, Sahiner Y. Exploring the feasibility of calculating expected pCO2 from venous blood gas samples alone in intensive care patients. Cureus. 2023;15(8):e42944. doi:10.7759/cureus.42944
Frequently asked questions
What is Winter formula: expected PaCO2 in metabolic acidosis?
Calculates the expected PaCO2 with its plus or minus 2 mmHg window and compares it with the measured value. Metabolic acidosis where the adequacy of the respiratory response is the question.
How is the result calculated?
Expected PaCO2 (mmHg) = 1.5 × bicarbonate (mmol/L) + 8, with a 2 mmHg window on each side.
How should the result be interpreted?
Winter formula applied step by step in blood gas interpretation, with the plus or minus 2 mmHg window and the reading of compensation.
What are the sources?
Muntingh J, Saffari S, Kelly A. Mastering blood gas interpretation: a practical guide for primary care providers. S Afr Fam Pract. 2025;67(1):6058. Turhan S, Tutan D, Sahiner Y. Exploring the feasibility of calculating expected pCO2 from venous blood gas samples alone in intensive care patients. Cureus. 2023;15(8):e42944.