Winter's Formula Calculator — Metabolic Acidosis ABG
Winter's formula predicts the PCO₂ a patient with metabolic acidosis should have if the lungs are compensating correctly. Compare it to the measured ABG value to identify mixed acid-base disorders.
mEq/L
mmHg
Winter's formula: 1.5 × HCO₃⁻ + 8 (±2 mmHg tolerance)
- 1
Multiply: 1.5 × HCO₃⁻
1.5 × 18 = 27 - 2
Expected PCO₂ = product + 8
27 + 8 = 35Winter's formula for metabolic acidosis: 1.5 × [HCO₃⁻] + 8 (±2 mmHg tolerance).
How does this calculator work?
Enter serum HCO₃⁻ (mEq/L) to get the expected PCO₂ via Winter's formula: 1.5 × HCO₃⁻ + 8 ± 2 mmHg. Compare to the measured ABG PCO₂ — if actual PCO₂ is below the range, concurrent respiratory alkalosis is present; if above, concurrent respiratory acidosis. For metabolic acidosis only.
Formula
How this is calculated
Winter's formula (published by Robert Winters in 1967) gives the expected arterial PCO₂ when a patient has metabolic acidosis and the respiratory system is providing appropriate compensation. As serum bicarbonate falls, the brain senses the acidaemia and drives the respiratory rate up, blowing off CO₂ to lower PCO₂ and thereby partly restore pH. The formula Expected PCO₂ = 1.5 × [HCO₃⁻] + 8 (with a ±2 mmHg tolerance band) captures this linear relationship.
If the measured PCO₂ from an arterial blood gas (ABG) falls within the predicted range, respiratory compensation is considered appropriate — the disorder is a pure metabolic acidosis. If the actual PCO₂ is lower than the expected range, the lungs are over-compensating, indicating a concurrent respiratory alkalosis (e.g. from pain, anxiety or pulmonary embolism). If the actual PCO₂ is higher than expected, the lungs are under-compensating, pointing to a concurrent respiratory acidosis (e.g. from COPD or sedation).
This formula applies only to metabolic acidosis — it should not be used for metabolic alkalosis or primary respiratory disorders. Normal serum bicarbonate is 22–26 mEq/L and normal arterial PCO₂ is 35–45 mmHg. Clinical interpretation must account for the full clinical picture, including pH, oxygenation, lactate, electrolytes and patient history. This tool is intended for educational use and clinical reference only.
Frequently asked questions
Winter's formula is used in ABG (arterial blood gas) interpretation to determine whether the respiratory system is compensating appropriately for a metabolic acidosis. By comparing the expected PCO₂ (1.5 × HCO₃⁻ + 8 ± 2 mmHg) to the measured PCO₂, clinicians can detect a concurrent respiratory alkalosis or acidosis in addition to the primary metabolic disorder.
Normal serum bicarbonate is 22–26 mEq/L. A value below 22 mEq/L, combined with an arterial pH below 7.35, indicates metabolic acidosis. The lower the bicarbonate, the more severe the acidosis, and the more the respiratory system needs to lower PCO₂ to compensate.
No. Winter's formula is valid only for metabolic acidosis. For metabolic alkalosis, the expected respiratory compensation is: Expected PCO₂ ≈ 0.7 × [HCO₃⁻] + 21 (±2 mmHg). This calculator implements only the metabolic acidosis variant.
Also known as
TG we-Calculate Editorial Team. (2026). Winter's Formula Calculator — Metabolic Acidosis ABG [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/winters-formula-calculator
TG we-Calculate Editorial Team. "Winter's Formula Calculator — Metabolic Acidosis ABG." TG we-Calculate. 2026. https://we-calculate.com/calculator/winters-formula-calculator.
TG we-Calculate Editorial Team, "Winter's Formula Calculator — Metabolic Acidosis ABG," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/winters-formula-calculator
@misc{wecalculate_winters_formula_calculator, title = {Winter's Formula Calculator — Metabolic Acidosis ABG}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/winters-formula-calculator}}, year = {2026}, note = {TG we-Calculate} }
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