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ABG Calculator (Arterial Blood Gas Interpreter)

Source: Castro D, Patil SM, Zubair M, Keenaghan M. Arterial Blood Gas Analysis. StatPearls, NCBI Bookshelf NBK536919 · Source verified August 22, 2026

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

An ABG (Arterial Blood Gas) calculator is an educational interpreter for acid-base physiology. It classifies pH as acidemic or alkalemic, identifies the primary disorder (respiratory vs. metabolic), calculates respiratory compensation using Winter's formula or metabolic alkalosis standards, assesses renal compensation, and computes the anion gap when electrolytes are provided.

Interpret arterial blood gas results to evaluate acid-base disorders, check for respiratory or metabolic compensation, and calculate the anion gap using electrolytes.

See also: pH Calculator, A1C Calculator, and Corrected Calcium Calculator.

Quick Answer

Interpret arterial blood gas (ABG) values. Enter pH, PaCO₂, and HCO₃⁻ to determine acidosis, alkalosis, compensation, and anion gap status.

Arterial Blood Gas Values

Normal: 7.35 - 7.45 · e.g. 7.40

Normal: 35 - 45 mmHg · e.g. 40

Normal: 22 - 26 mEq/L · e.g. 24

Oxygenation (Optional)

Normal: 75 - 100 mmHg (StatPearls)

Anion Gap Electrolytes (Optional)

Normal: 135 - 145

Normal: 98 - 107

ABG Interpretation Results

Interpretation

Normal Acid-Base Status

No mixed respiratory/metabolic compensation required

pH status7.40 (Normal)
PaCO₂40 mmHg
HCO₃⁻24 mEq/L

Bands this tool compares against: pH 7.35-7.45, PaCO₂ 35-45 mmHg, HCO₃⁻ 22-26 mEq/L (StatPearls). Accredited laboratories set their own ranges; yours is the one that applies.

Disclaimer: This calculator is for educational and training purposes only. It must not be used to direct patient care or substitute for clinical judgment.

Step-by-Step Diagnostic Steps

[1]Classify the pH: Actual pH = 7.40 (Reference range: 7.35 – 7.45)
[2] - This indicates NORMAL.
[3]Identify the primary disturbance:
[4] - Primary assessment indicates: Normal Acid-Base Status
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Examples

Metabolic acidosis with respiratory compensation: pH 7.20, PaCO2 25, HCO3 10

Acidosis · Metabolic · Expected PaCO2 = 1.5(10)+8 = 23 ± 2 (Actual 25 is compensated)

Respiratory acidosis: pH 7.28, PaCO2 55, HCO3 26

Acidosis · Respiratory · HCO3 is near normal (indicative of acute decompensation)

Mixed metabolic & respiratory alkalosis: pH 7.55, PaCO2 30, HCO3 30

Alkalosis · Combined Respiratory Alkalosis (low PaCO2) and Metabolic Alkalosis (high HCO3)

How it works

The interpretation of an ABG sample follows a structured diagnostic sequence:

  1. Look at the pH: Determine if there is acidemia (<7.35) or alkalemia (>7.45).
  2. Identify the primary driver: If pH is acidotic, look for a high PaCO₂ (respiratory) or a low HCO₃⁻ (metabolic). If alkalotic, look for a low PaCO₂ (respiratory) or a high HCO₃⁻ (metabolic).
  3. Evaluate compensation: Calculate the expected secondary response. For metabolic acidosis, we use Winter's formula.
  4. Check the anion gap (optional): An elevated anion gap indicates the presence of unmeasured anions (HAGMA).

Clinical Context & Anion Gap

For patient care, an elevated anion gap (above 12 mEq/L) is a vital clue in metabolic acidosis, pointing to causes like lactic acidosis, diabetic ketoacidosis, or ingestion of toxic substances (MUDPILES). Normal anion gap metabolic acidosis (NAGMA) is typically caused by bicarbonate loss, such as through diarrhea or renal tubular acidosis. Always correlate blood gas values with the patient's clinical presentation, history, and physical exam.

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Frequently asked questions

An Arterial Blood Gas (ABG) test measures the levels of oxygen, carbon dioxide, and bicarbonate in the blood, as well as blood pH. It is a critical tool used to evaluate respiratory function and identify metabolic and renal disturbances in acid-base balance.

The ranges this tool compares against are pH 7.35 to 7.45, PaCO₂ 35 to 45 mmHg and HCO₃⁻ 22 to 26 mEq/L, which match what StatPearls publishes for arterial blood gas analysis. Two caveats belong with them. First, there is no centrally published set: laboratories accredited by CAP, CLIA, COLA or the FDA are required to establish or validate their own reference ranges, and StatPearls itself notes that values vary among laboratories and across age groups. Second, PaO₂ is different in kind. StatPearls publishes 75 to 100 mmHg, not the 80 to 100 this page used to state, and PaO₂ falls with age: Sorbini's 1968 formula, PaO₂ = 109 − (0.43 × age), predicts about 75 mmHg at age 80, so a fixed adult floor flags healthy older people. This tool does not classify PaO₂ for that reason.

When a primary respiratory or metabolic disturbance occurs, the body attempts to return pH toward normal. Respiratory disorders are compensated by the kidneys adjusting HCO₃⁻ excretion (takes 2–5 days). Metabolic disorders are compensated quickly by the lungs adjusting ventilation to change PaCO₂ (takes minutes to hours).

Winter's formula is a mathematical model used in metabolic acidosis to calculate the expected PaCO₂ needed for adequate respiratory compensation: Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2. If the actual PaCO₂ is higher than expected, a primary respiratory acidosis is also present; if lower, a primary respiratory alkalosis is present.