Winters Formula CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimate expected respiratory compensation in metabolic acidosis.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
Sign in to save results and favorites.
About the Winters Formula Calculator
The Winters Formula Calculator estimates the expected arterial carbon dioxide partial pressure (PaCO₂) during respiratory compensation for metabolic acidosis. It uses serum bicarbonate to calculate an expected PaCO₂ and a range of ±2 mmHg, which can be compared with a measured arterial PaCO₂ to assess whether an additional respiratory acid–base disorder may be present. This tool provides an educational estimate for adults, not a diagnosis or treatment recommendation.
- Serum bicarbonate (mmol/L)
- Serum bicarbonate is the concentration of bicarbonate (HCO₃⁻) in the blood, entered in millimoles per liter (mmol/L), with a default of 16.
Formula and method
P = 1.5 × B + 8; L = P − 2; U = P + 2. With B = 16 mmol/L: P = 1.5 × 16 + 8 = 32 mmHg; L = 30 mmHg; U = 34 mmHg.
- BSerum bicarbonate concentration in mmol/L.
- PExpected arterial carbon dioxide partial pressure (PaCO₂) in mmHg.
- LLower bound of the expected PaCO₂ range in mmHg.
- UUpper bound of the expected PaCO₂ range in mmHg.
Multiply the serum bicarbonate value by 1.5 and add 8 to estimate the expected PaCO₂ in mmHg. Subtract and add 2 mmHg to obtain the expected lower and upper bounds. The coefficients assume bicarbonate is entered in mmol/L; the calculator estimates compensation rather than measuring PaCO₂ or establishing the presence of metabolic acidosis.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Serum bicarbonate16 mmol/L
Results
- Expected PaCO₂32 mmHg
- Expected lower bound30 mmHg
- Expected upper bound34 mmHg
- NoteEducational estimate for adults; not a diagnosis or treatment recommendation.
How to use the Winters Formula Calculator
- 1Use this tool in the context of metabolic acidosis established through clinical and laboratory assessment.
- 2Enter serum bicarbonate in mmol/L, or retain the default value of 16 for the worked example.
- 3Run the calculation to obtain the expected PaCO₂ and its lower and upper bounds.
- 4For the default input, review Expected PaCO₂ = 32 mmHg, Expected lower bound = 30 mmHg, and Expected upper bound = 34 mmHg.
- 5If an arterial blood gas result is available, compare its measured PaCO₂ with the expected range and seek clinical interpretation rather than using the estimate to make treatment decisions.
Reading your result
Expected PaCO₂ is the central estimate of respiratory compensation: 32 mmHg with the default bicarbonate of 16 mmol/L. Expected lower bound is 30 mmHg, and Expected upper bound is 34 mmHg. A measured arterial PaCO₂ within this range is consistent with expected respiratory compensation, but does not establish a normal pH or exclude other metabolic disorders. A measured value above the upper bound suggests an additional respiratory acidosis; a value below the lower bound suggests an additional respiratory alkalosis. The Note row reads: Educational estimate for adults; not a diagnosis or treatment recommendation.
Limitations
Winter’s formula is an empirical estimate for metabolic acidosis, not a general acid–base screening formula or a tool for metabolic alkalosis. Low bicarbonate alone does not establish metabolic acidosis, because it can also occur with respiratory alkalosis. Interpretation requires clinical context and usually blood gas and electrolyte results; respiratory compensation may be incomplete in rapidly evolving illness or affected by lung disease and mechanical ventilation. The ±2 mmHg range is an expected compensation band, not a guaranteed patient-specific interval. Venous PCO₂ is not interchangeable with arterial PaCO₂ for this comparison. This adult educational tool does not identify the cause of acidosis, assess all mixed metabolic disorders, or set ventilation or treatment targets.
Common questions
Why does bicarbonate of 16 mmol/L produce an expected PaCO₂ of 32 mmHg?+
Winter’s formula calculates 1.5 × 16 + 8 = 32 mmHg. Applying the ±2 mmHg allowance gives an expected range of 30–34 mmHg.
Does the calculator measure my actual PaCO₂?+
No. It predicts the PaCO₂ expected from respiratory compensation for metabolic acidosis. Actual arterial PaCO₂ must be measured with an arterial blood gas test.
What does a measured PaCO₂ outside 30–34 mmHg mean for the default input?+
With bicarbonate of 16 mmol/L and established metabolic acidosis, a measured arterial PaCO₂ above 34 mmHg suggests additional respiratory acidosis, while a value below 30 mmHg suggests additional respiratory alkalosis. These findings require clinical interpretation.
Can Winter’s formula be used whenever bicarbonate is low?+
Not without establishing the acid–base context. Low bicarbonate may reflect metabolic acidosis or compensation for respiratory alkalosis; Winter’s formula is intended to assess respiratory compensation specifically in metabolic acidosis.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
You might also like
Macro Calculator
Daily protein, carbs and fat for your goal.
Carbohydrate Calculator
Daily carb range (45–65% of calories).
Protein Calculator
Daily protein needs by weight and activity.
Fat Intake Calculator
Healthy daily fat range.
TDEE Calculator
Total daily energy expenditure.
GFR Calculator
Kidney function estimate (CKD-EPI 2021).
Results are estimates for informational purposes and are not financial, medical or legal advice.