Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Expected pCO2
26.0
Expected range (±2)
24.0–28.0
Compensation assessment
Appropriate respiratory compensation
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How to use this
- 1Enter bicarbonate (hco3) (mEq/L).
- 2Enter measured pco2 (mmHg).
- 3Read your expected pco2 on the right — it updates as you type.
- 4Hit Share to keep the scenario or send it to someone.
About this calculator
Winter's formula predicts the expected pCO2 for a given degree of metabolic acidosis, letting you check whether a patient's respiratory system is compensating appropriately or whether a second, independent acid-base disorder is also present. If the measured pCO2 matches the predicted range, the respiratory response is appropriate and there is a single primary metabolic acidosis. If measured pCO2 is lower than predicted, there is an additional primary respiratory alkalosis (the patient is hyperventilating beyond what compensation alone would explain — classic in sepsis, salicylate toxicity, or pain/anxiety). If measured pCO2 is higher than predicted, there is an additional primary respiratory acidosis (the patient cannot compensate adequately, from fatigue, sedation, opioids, or an intrinsic lung/neuromuscular process) — this is a dangerous combination because there's no more respiratory buffering left. Winter's formula only applies to metabolic acidosis; different expected-compensation formulas exist for the other three primary acid-base disorders. This tool is for education and workflow support only — not medical advice. Always verify results with a clinician and use clinical judgment alongside your institution's protocols.
Worked example
Using the values the calculator loads with:
Inputs
- Bicarbonate (HCO3): 12 mEq/L
- Measured pCO2: 26 mmHg
Results
- Expected pCO2: 26
- Expected range (±2): 24.0–28.0
- Compensation assessment: Appropriate respiratory compensation
What each field means
Inputs
- Bicarbonate (HCO3) (mEq/L)
- The bicarbonate (hco3) used in the calculation, measured in mEq/L. Starts at 12 mEq/L so you have a working example on load. Accepted range: 2–40 mEq/L.
- Measured pCO2 (mmHg)
- The measured pco2 used in the calculation, measured in mmHg. Starts at 26 mmHg so you have a working example on load. Accepted range: 5–100 mmHg.
Results
- Expected pCO2
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Expected range (±2)
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Compensation assessment
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
What if measured pCO2 falls within the expected range?
Then the respiratory system is compensating exactly as predicted for a single, simple metabolic acidosis, and you don't need to look for a second acid-base process based on this test alone. Continue treating the underlying cause of the metabolic acidosis.
What does it mean if the pCO2 is lower than predicted?
It means the patient is hyperventilating beyond what compensation for the metabolic acidosis alone would explain, indicating an additional primary respiratory alkalosis. Common causes include sepsis, pain, anxiety, pregnancy, and salicylate toxicity, which classically causes both a primary respiratory alkalosis and an anion gap metabolic acidosis simultaneously.
Why is a higher-than-predicted pCO2 particularly concerning?
It means the patient cannot mount adequate respiratory compensation for their metabolic acidosis — there's a concurrent primary respiratory acidosis on top of the metabolic one, so both systems are failing to protect pH. This combination causes a much more severe acidemia than either disorder alone and often signals respiratory fatigue, sedation, or a separate lung process requiring urgent attention.
Does Winter's formula apply to other acid-base disorders?
No — it's specific to compensation for metabolic acidosis. Metabolic alkalosis, respiratory acidosis, and respiratory alkalosis each have their own separate expected-compensation formulas and time courses (acute versus chronic respiratory disorders compensate differently), so don't apply Winter's formula outside its intended use.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Winter's Formula Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/winters-formula
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/winters-formula" target="_blank" rel="noopener">Winter's Formula Calculator — RevenueLab</a> (2026).</p>
Source: [Winter's Formula Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/winters-formula) (2026).