Last updated October 10, 2026 · Report an error
Winters' Formula (Winters RW et al., "Disturbances of acid-base equilibrium", chapter in "The Body Fluids in Pediatrics", 1967; commonly attributed to work from Winters and colleagues) estimates the expected respiratory compensation for a metabolic acidosis: Expected PaCO2 = 1.5 x HCO3 + 8, plus or minus 2. Comparing a patient's actual measured PaCO2 to this expected value can suggest whether a coexisting respiratory disorder is also present, but this tool only computes the expected reference value from HCO3 — it does not interpret or compare an actual PaCO2 for you. For general reference and educational purposes only — not a medical diagnosis. Consult a doctor for actual acid-base assessment.
How it works
Enter serum bicarbonate (HCO3) in mEq/L. The tool applies Winters' formula to compute the expected PaCO2 and its ±2 mmHg range.
- Enter serum bicarbonate, HCO3 (mEq/L).
- Click Calculate to see your results.
Examples
HCO3 10 mEq/L
Expected PaCO2 = 1.5 x 10 + 8 = 23 mmHg (range: 21-25 mmHg).
Who should use it
- Reviewing an arterial blood gas alongside a doctor.
- Emergency medicine and critical care education about acid-base compensation.
Industry applications
- Emergency medicine and critical care reference
- Nephrology and pulmonology education
Advantages
- Long-established, widely taught formula.
- Simple — needs only the bicarbonate value.
Limitations
- Only estimates the expected value — does not interpret the patient's actual measured PaCO2.
- A reference calculation, not a standalone diagnostic tool.
Common mistakes to avoid
- Comparing the expected PaCO2 to a venous rather than arterial blood gas value.
- Forgetting the standard ±2 mmHg tolerance range when comparing to an actual measured PaCO2.
Best practices
- Always compare the expected PaCO2 against an actual arterial blood gas PaCO2, with the ±2 mmHg range in mind, alongside a doctor's interpretation.
Tips
- If the actual measured PaCO2 falls well outside the expected ±2 mmHg range, that is typically a prompt to consider an additional respiratory acid-base disorder, per commonly cited teaching.