Arterial Blood pH Calculator
Calculate arterial blood pH from bicarbonate and PaCO₂ using the Henderson-Hasselbalch equation. Evaluate acidosis, alkalosis, and acid-base status with detailed interpretation.
About This Calculator
What is Arterial Blood Gas pH?
Arterial blood pH measures the acidity or alkalinity of arterial blood and is one of the most critical parameters in clinical medicine. It is obtained through arterial blood gas (ABG) analysis, which also measures PaO₂ (oxygen partial pressure), PaCO₂ (carbon dioxide partial pressure), bicarbonate (HCO₃⁻), and base excess. The normal arterial blood pH range is 7.35–7.45. A pH below 7.35 defines acidemia, while a pH above 7.45 defines alkalemia. These conditions reflect underlying acid-base disorders that can be respiratory (driven by CO₂) or metabolic (driven by HCO₃⁻). This calculator is used by physicians, residents, medical students, nurses, and respiratory therapists in emergency departments, ICUs, and pulmonary clinics.
How is Arterial Blood pH Calculated?
This calculator uses the Henderson-Hasselbalch equation, the gold standard formula for calculating blood pH from bicarbonate and PaCO₂:
pH = 6.1 + log₁₀[ HCO₃⁻ / (0.0308 × PaCO₂) ]
The constant 6.1 is the pKa of carbonic acid (H₂CO₃) at body temperature (37°C). The constant 0.0308 is the solubility coefficient of CO₂ in blood plasma (mmol/L per mmHg). Bicarbonate is entered in mEq/L (numerically equivalent to mmol/L), and PaCO₂ is entered in mmHg. For example, with normal values of HCO₃⁻ = 24 mEq/L and PaCO₂ = 40 mmHg: pH = 6.1 + log₁₀[24 / (0.0308 × 40)] = 6.1 + log₁₀(19.48) = 6.1 + 1.289 = 7.39, which falls within the normal range.
Clinical Interpretation of Acid-Base Disorders
Once the pH is calculated, the next step is to determine whether the disorder is respiratory or metabolic, and whether compensation is present. Respiratory acidosis (pH < 7.35, PaCO₂ > 45 mmHg) is caused by hypoventilation from COPD, opioid overdose, pneumonia, asthma, or neuromuscular weakness. Respiratory alkalosis (pH > 7.45, PaCO₂ < 35 mmHg) is caused by hyperventilation from anxiety, pulmonary embolism, high altitude, or salicylate toxicity. Metabolic acidosis (pH < 7.35, HCO₃⁻ < 22 mEq/L) is caused by DKA, lactic acidosis, renal failure, diarrhea, or toxin ingestion. Metabolic alkalosis (pH > 7.45, HCO₃⁻ > 26 mEq/L) is caused by vomiting, nasogastric suction, diuretics, or hyperaldosteronism. Winter's formula helps assess appropriate respiratory compensation in metabolic acidosis.
Regional Notes
India: Arterial blood gas analysis is widely available in tertiary care hospitals, medical college hospitals, and many district-level facilities across India. The same international reference ranges (pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L) are used in Indian clinical practice. ABG analysis is essential in managing diabetic ketoacidosis (common in the Indian population), COPD exacerbations (high prevalence due to tobacco use and biomass fuel exposure), and tropical infections causing sepsis or respiratory failure.
US: ABG analysis is a standard diagnostic test in every US hospital emergency department and intensive care unit. The American Thoracic Society (ATS) and the American College of Chest Physicians (CHEST) provide detailed guidelines for ABG interpretation. US reference ranges match the international standard. The Henderson-Hasselbalch equation is taught in all US medical schools and residency programs.
UK: The National Health Service (NHS) performs ABG analysis routinely across its intensive care, respiratory, and emergency medicine departments. British Thoracic Society (BTS) guidelines recommend ABG interpretation for all patients with acute respiratory failure. UK reference ranges (pH 7.35–7.45) are identical to US and Indian standards.
Important Disclaimer
This calculator provides reference information only. Arterial blood pH interpretation should always be performed by a qualified healthcare professional as part of a comprehensive clinical assessment, including full ABG analysis and correlation with patient history. It is not a substitute for professional medical advice, diagnosis, or treatment.
Frequently Asked Questions
What is arterial blood pH and why is it measured?
Arterial blood pH measures the acidity or alkalinity of arterial blood. It is a critical component of arterial blood gas (ABG) analysis used to assess acid-base balance in the body. Normal arterial blood pH ranges from 7.35 to 7.45. A pH below 7.35 indicates acidemia (excess acid in the blood), while a pH above 7.45 indicates alkalemia (excess base). ABG analysis helps diagnose respiratory and metabolic disorders affecting acid-base homeostasis and is routinely used in ICUs, emergency departments, and pulmonary medicine across India, the US, and the UK.
How is arterial blood pH calculated using the Henderson-Hasselbalch equation?
The Henderson-Hasselbalch equation calculates arterial blood pH from bicarbonate (HCO₃⁻) and arterial carbon dioxide partial pressure (PaCO₂): pH = 6.1 + log₁₀[HCO₃⁻ / (0.0308 × PaCO₂)]. The constant 0.0308 represents the solubility coefficient of CO₂ in blood at 37°C (mmol/L per mmHg). This equation is the gold standard for acid-base assessment in clinical medicine and was developed by Dr. Lawrence Joseph Henderson in 1908 and refined by Dr. Karl Albert Hasselbalch.
What is the normal range for arterial blood pH?
The normal range for arterial blood pH is 7.35–7.45. Venous blood is slightly more acidic with a normal range of 7.31–7.41. Normal bicarbonate (HCO₃⁻) levels range from 22–26 mEq/L, and normal PaCO₂ ranges from 35–45 mmHg. If the calculated pH is below 7.35, it indicates acidosis (respiratory or metabolic). If above 7.45, it indicates alkalosis. These reference ranges are used consistently in clinical practice across India, the United States, and the United Kingdom.
What is the difference between respiratory and metabolic acidosis?
Respiratory acidosis is caused by CO₂ retention from hypoventilation (PaCO₂ > 45 mmHg), which drives the pH down. Common causes include COPD exacerbation, opioid overdose, pneumonia, and neuromuscular disorders. Metabolic acidosis is caused by a primary decrease in bicarbonate (HCO₃⁻ < 22 mEq/L) from conditions such as diabetic ketoacidosis (DKA), lactic acidosis, renal failure, diarrhea, or toxin ingestion. The Henderson-Hasselbalch equation helps differentiate these by analyzing whether the pH change is driven by PaCO₂ (respiratory) or HCO₃⁻ (metabolic).
What is the difference between respiratory and metabolic alkalosis?
Respiratory alkalosis is caused by CO₂ elimination from hyperventilation (PaCO₂ < 35 mmHg). Causes include anxiety/panic attacks, pulmonary embolism, high altitude, salicylate overdose, and mechanical overventilation. Metabolic alkalosis is caused by a primary increase in bicarbonate (HCO₃⁻ > 26 mEq/L) from conditions such as vomiting, nasogastric suction, diuretic therapy, hyperaldosteronism, or alkali ingestion. The body compensates for these imbalances through renal and respiratory mechanisms.
What is compensated vs uncompensated acid-base disorder?
An uncompensated disorder shows an abnormal pH with the primary parameter (PaCO₂ for respiratory, HCO₃⁻ for metabolic) outside its normal range. In a compensated disorder, the pH returns toward normal (7.35–7.45) because the opposing system adjusts — renal compensation for respiratory disorders (hours to days) or respiratory compensation for metabolic disorders (minutes to hours). For example, in chronic respiratory acidosis from COPD, the kidneys retain bicarbonate to raise pH toward normal. Full compensation does not always return pH to exactly 7.40, but it often reaches the normal range.
What is the anion gap and how does it relate to arterial blood pH?
The anion gap (AG = Na⁺ − [Cl⁻ + HCO₃⁻]) is a complementary test that helps classify metabolic acidosis. A high anion gap metabolic acidosis (HAGMA) with low pH indicates accumulation of unmeasured anions such as lactate (sepsis, shock), ketones (DKA), or toxins (methanol, ethylene glycol, salicylates). A normal anion gap metabolic acidosis (NAGMA, hyperchloremic) with low pH indicates bicarbonate loss from diarrhea or renal tubular acidosis. The arterial blood pH calculator and anion gap calculator together provide a comprehensive acid-base assessment.
Can I rely on this arterial blood pH calculator for clinical decisions?
This arterial blood pH calculator is designed for educational and reference purposes. While it uses the standard Henderson-Hasselbalch equation validated in clinical medicine, results should always be interpreted by a qualified healthcare professional in the context of the full clinical picture — including patient history, physical examination, arterial blood gas sampling, and additional laboratory studies. In India, the United States, and the United Kingdom, acid-base disorders are diagnosed and managed by licensed physicians. This tool is not a substitute for professional medical advice, diagnosis, or treatment.