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Acid–Base Compensation Calculator

Choose the primary acid–base disorder to get the expected compensation. Enter the measured compensating value and the tool tells you whether compensation is appropriate or a second disorder is present.

Try:
Expected PaCO₂ 26 mmHg
Range 24–28

For education and clinical decision support only. These compensation rules are approximations with published ranges and must be read alongside the arterial pH, the anion gap and the full clinical picture. This tool does not replace professional medical judgement.

What is acid–base compensation?

Compensation is the body's secondary defence of the blood pH. When a primary acid–base disorder pushes the pH one way, a second system pushes back: a metabolic disorder is compensated by the lungs adjusting PaCO₂, and a respiratory disorder is compensated by the kidneys adjusting bicarbonate. Compensation blunts the pH change but never fully corrects it — so an over- or under-shoot signals a second, mixed disorder.

The compensation formulas

  • Metabolic acidosis — expected PaCO₂ = 1.5 × HCO₃ + 8 ± 2 (Winters' formula).
  • Metabolic alkalosis — expected PaCO₂ = 0.7 × HCO₃ + 20 ± 5.
  • Acute respiratory acidosis — HCO₃ rises about 1 mEq/L per 10 mmHg rise in PaCO₂ above 40.
  • Chronic respiratory acidosis — HCO₃ rises about 4 mEq/L per 10 mmHg rise in PaCO₂ (some texts use 3.5).
  • Acute respiratory alkalosis — HCO₃ falls about 2 mEq/L per 10 mmHg fall in PaCO₂ below 40.
  • Chronic respiratory alkalosis — HCO₃ falls about 4 mEq/L per 10 mmHg fall in PaCO₂ (some texts use 5).

How to read the result

Enter the primary value and the tool gives the expected compensating value with a tolerance range. Add the measured compensating value to get a verdict:

  • Within range — compensation is appropriate; a single, pure disorder.
  • Outside range — a concurrent (mixed) disorder is present, named in the interpretation.

Why acute and chronic differ

Renal compensation for a respiratory disorder is slow, taking 2–5 days to reach its full effect. Immediately after PaCO₂ changes, only a small acute shift in bicarbonate occurs; over days the kidneys make a much larger adjustment. That is why respiratory acidosis and alkalosis each have separate acute and chronic expected values.

Frequently asked questions

What are the acid–base compensation formulas?

Metabolic acidosis: expected PaCO₂ = 1.5 × HCO₃ + 8 ± 2 (Winters'). Metabolic alkalosis: expected PaCO₂ = 0.7 × HCO₃ + 20 ± 5. Acute respiratory acidosis: HCO₃ rises 1 per 10 mmHg rise in PaCO₂; chronic, about 4 per 10. Acute respiratory alkalosis: HCO₃ falls 2 per 10 mmHg fall in PaCO₂; chronic, about 4 per 10.

What is compensation in acid–base balance?

Compensation is the body's secondary response that limits the change in pH from a primary disorder. A metabolic problem is compensated by the lungs changing PaCO₂; a respiratory problem is compensated by the kidneys changing bicarbonate. Compensation never fully corrects the pH.

How do you know if compensation is appropriate?

Calculate the expected compensating value for the primary disorder and compare it with the measured value. If the measured value matches the prediction, compensation is appropriate. If it does not, a second (mixed) acid–base disorder is present.

What is the difference between acute and chronic respiratory compensation?

Renal (metabolic) compensation for a respiratory disorder takes 2–5 days to develop fully. Acute changes reflect immediate buffering, while chronic changes reflect the larger renal adjustment, so the expected bicarbonate is different for acute versus chronic respiratory acidosis or alkalosis.

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