GuidesFree toolsFree practicePricing

TidalPrep › Free tools › ABG interpretation calculator

ABG interpretation calculator

Study aid only. Not for clinical decisions. This calculator teaches textbook formulas for exam prep. It is not a medical device and does not replace clinical judgment, your facility's protocols, or a licensed clinician's assessment. Nothing you enter leaves your browser.

How it works

  1. pH: below 7.35 is acidemia and above 7.45 is alkalemia. Normal PaCO2 is 35–45 mm Hg and normal HCO3− is 22–26 mEq/L.[1][2]
  2. Primary disorder: the value that matches the pH change. A high PaCO2 with acidemia is respiratory; a low HCO3− with acidemia is metabolic.
  3. Compensation (classic RT classification): uncompensated if the other value is still normal, partially compensated if it has moved but the pH is still abnormal, fully compensated if the pH is back to 7.35–7.45. With a normal pH, the side of 7.40 the pH falls on points to the primary process.
  4. Expected compensation (if the value is outside the range, a second disorder is present):[6][1]
    • Metabolic acidosis: expected PaCO2 = 1.5 × HCO3− + 8 ± 2 (Winter's formula)[3]
    • Metabolic alkalosis: expected PaCO2 ≈ 0.7 × HCO3− + 20 ± 5[4]
    • Respiratory acidosis: HCO3− rises about 1 mEq/L per 10 mm Hg PaCO2 (acute) or about 3.5 (chronic)[5][6]
    • Respiratory alkalosis: HCO3− falls about 2 mEq/L per 10 mm Hg (acute) or 4–5 (chronic)[6]
  5. Anion gap = Na+ − (Cl− + HCO3−). Compare it with your lab's reference range.[7]

Worked example

pH 7.28, PaCO2 29, HCO3− 14: acidemia with a low HCO3− means metabolic acidosis, and the low PaCO2 means it is partially compensated. Winter's formula: 1.5 × 14 + 8 = 29 ± 2, so the respiratory compensation is appropriate. If the PaCO2 had been 38, a respiratory acidosis would also be present.

Practice it now. Drill these calculations as exam questions: 50 free practice items with full rationales, no account needed.
Start free: 50 questions

Sources

  1. American Thoracic Society: Interpretation of Arterial Blood Gases
  2. StatPearls: Arterial Blood Gas
  3. Merck Manual: Winter's Formula for Expected PCO2
  4. StatPearls: Metabolic Alkalosis
  5. StatPearls: Respiratory Acidosis
  6. Merck Manual: Primary Changes and Compensations in Simple Acid-Base Disorders (table)
  7. StatPearls: Anion Gap and Non-Anion Gap Metabolic Acidosis

Formulas checked against the sources listed above on October 30, 2026. Written and reviewed by a practicing RRT for TidalPrep.