Winter's formula predicts the expected pCO₂ in metabolic acidosis. If the measured pCO₂ doesn't match, a second respiratory disorder is present. Enter your values below.
Enter bicarbonate and measured pCO₂ to check respiratory compensation.
From metabolic panel and arterial blood gas
Enter values and click Calculate
The standard formula for predicting respiratory compensation.
Multiply the bicarbonate by 1.5, add 8, and allow a window of ±2 mmHg. This gives the expected pCO₂ if the respiratory system is compensating normally for a primary metabolic acidosis.
A quick bedside check: in a simple metabolic acidosis, the pCO₂ should approximately equal the last two digits of the pH. For example, pH 7.25 → expected pCO₂ around 25 mmHg. This doesn't replace Winter's formula but helps with rapid assessment.
Updates in real-time.
How the lungs compensate for metabolic acidosis — and how to check if they're doing it right.
Winter's formula predicts how much the lungs should lower pCO₂ in response to metabolic acidosis. When blood becomes acidic (low bicarbonate), the brainstem drives faster breathing to blow off CO₂ and raise the pH back toward normal. This compensation is predictable and follows a linear relationship described by Winter's formula.
If the measured pCO₂ matches the predicted range, you have a simple metabolic acidosis with appropriate compensation. If pCO₂ is lower than expected, the patient has a concurrent respiratory alkalosis (perhaps from sepsis, PE, or pain). If pCO₂ is higher than expected, the patient has a concurrent respiratory acidosis (respiratory failure, sedation, or neuromuscular weakness). This changes management significantly.
| HCO₃⁻ (mEq/L) | Expected pCO₂ (mmHg) | Range (±2) |
|---|---|---|
| 5 | 15.5 | 13.5 – 17.5 |
| 10 | 23.0 | 21.0 – 25.0 |
| 15 | 30.5 | 28.5 – 32.5 |
| 20 | 38.0 | 36.0 – 40.0 |
| 24 (normal) | 44.0 | 42.0 – 46.0 |
Gauge shows measured pCO₂ relative to expected range.
What the comparison between expected and measured pCO₂ reveals.