A complete, free library of respiratory therapy calculators covering oxygenation, ventilation, acid-base balance, hemodynamics, pulmonary function, and mechanical ventilation.
Each tool helps you apply clinical formulas quickly and accurately, whether you’re preparing for the board exam or working at the bedside. Search by name or browse by category to find what you need.
Acid-Base & Blood Gases 5
Arterial Blood Gas (ABG)
Interpret pH, PaCO₂, HCO₃⁻, and oxygenation to identify acid-base disorders.
Venous Blood Gas (VBG)
Assess acid-base status and ventilation trends from a venous sample.
Anion Gap
Evaluate metabolic acidosis from sodium, chloride, and bicarbonate.
Base Excess (BE)
Gauge the metabolic component of an acid-base disorder.
Winters' Formula
Check expected respiratory compensation for metabolic acidosis.
Oxygenation & Oxygen Transport 13
Alveolar-Arterial (A-a) Gradient
Compare alveolar and arterial oxygen to classify hypoxemia.
Arterial Oxygen Content (CaO₂)
Total oxygen carried in arterial blood.
SaO₂ Estimation
Estimate hemoglobin saturation from PaO₂.
Desired FiO₂
Find the FiO₂ needed to reach a target PaO₂.
End-Capillary O₂ Content (CcO₂)
Oxygen content after ideal alveolar gas exchange.
Mixed Venous O₂ Content (CvO₂)
Oxygen remaining in venous blood after tissue extraction.
Oxygen Consumption (VO₂)
Oxygen used by the body each minute.
Oxygen Content Difference (C(a-v)O₂)
Arterial versus mixed venous oxygen content.
Oxygen Extraction Ratio (O₂ER)
Percent of delivered oxygen used by the tissues.
Oxygenation Index (OI)
Oxygenation severity from FiO₂, mean airway pressure, and PaO₂.
PaO₂/FiO₂ (P/F) Ratio
Oxygenation efficiency and severity of hypoxemia.
Shunt Equation (Qₛ/Qₜ)
Fraction of cardiac output that bypasses oxygenation.
Modified Shunt Equation
Shunt estimate using a simplified approach.
Ventilation & CO₂ Removal 9
Minute Ventilation
Total gas moved each minute from tidal volume and rate.
Alveolar Minute Ventilation (VA)
Fresh gas reaching the alveoli each minute.
Corrected Minute Ventilation
Ventilatory adjustment when PaCO₂ is off target.
Tidal Volume (VT)
Air inhaled or exhaled with each breath.
Corrected Tidal Volume
Delivered volume after leaks and circuit compliance.
Exhaled Tidal Volume
Air leaving the lungs during each breath.
Dead Space/Tidal Volume Ratio
Portion of each breath that is wasted ventilation.
Target PaCO₂ Adjustment
New minute ventilation needed to reach a PaCO₂ goal.
Respiratory Quotient (RQ)
CO₂ production relative to oxygen consumption.
Ventilator Mechanics & Timing 20
Airway Resistance (Raw)
Pressure needed to move gas through the airways.
Auto-PEEP
Unintended end-expiratory pressure from air trapping.
Driving Pressure (ΔP)
Pressure delivering tidal volume above PEEP.
Dynamic Compliance (Cdyn)
Lung and airway expansion during active airflow.
Static Compliance (Cstat)
Lung and chest wall expansion with airflow paused.
Elastance
The respiratory system's tendency to resist expansion.
Mechanical Power (MP)
Total energy delivered to the respiratory system.
Mean Airway Pressure (Paw)
Average pressure across the full respiratory cycle.
Plateau Pressure (Pplat)
Alveolar pressure during an inspiratory hold.
Transpulmonary Pressure (PL)
Distending pressure across the lung.
Time Constant
How quickly the lungs fill and empty.
I:E Ratio
Inspiratory time compared with expiratory time.
Inspiratory Time (Ti)
Time spent delivering or taking in a breath.
Expiratory Time (Te)
Time available for exhalation each breath.
Peak Inspiratory Flow (PIF)
Highest flow reached during inspiration.
Minimum Flow Rate
Flow needed to deliver tidal volume within the set Ti.
Pressure Support (PSV)
Inspiratory pressure that reduces work of breathing.
Work of Breathing (WOB)
Effort required to move air in and out of the lungs.
Rapid Shallow Breathing Index (RSBI)
Rate-to-tidal-volume ratio for weaning readiness.
ETT Depth & Tidal Volume
Airway placement and ventilator volume estimates.
Lung Volumes & Pulmonary Function 9
Expiratory Reserve Volume (ERV)
Extra air exhaled after a normal exhalation.
Functional Residual Capacity (FRC)
Air remaining after a normal passive exhalation.
Inspiratory Capacity (IC)
Maximum air inhaled after a normal exhalation.
Residual Volume (RV)
Air remaining after a maximal exhalation.
Total Lung Capacity (TLC)
Maximum air the lungs can hold after full inspiration.
Vital Capacity (VC)
Maximum air exhaled after the deepest breath.
FEF25–75%
Mid-expiratory flow through the smaller airways.
FEV1/FVC Ratio
Key value for identifying obstructive lung disease.
Predicted Peak Expiratory Flow
Expected PEF from age, sex, and height.
Hemodynamics & Circulation 10
Cardiac Output (QT)
Blood pumped by the heart each minute.
Cardiac Index (CI)
Cardiac output indexed to body surface area.
Fick's Method Cardiac Output
Output from oxygen consumption and content difference.
Stroke Volume (SV)
Blood ejected by the left ventricle per heartbeat.
Mean Arterial Pressure (MAP)
Average arterial pressure during one cardiac cycle.
Cerebral Perfusion Pressure (CPP)
Pressure gradient driving blood flow to the brain.
Pulmonary Vascular Resistance (PVR)
Resistance the right ventricle overcomes through the lungs.
Systemic Vascular Resistance (SVR)
Resistance the left ventricle overcomes through the body.
Electrocardiogram (ECG) Heart Rate
Heart rate from R-wave spacing on an ECG.
Maximum Heart Rate (HRmax)
Estimated upper heart-rate limit for activity.
Oxygen Delivery & Equipment 5
FiO₂ for Nasal Cannula
Approximate FiO₂ delivered at different flow rates.
O₂-to-Air Entrainment Ratio
Room-air-to-oxygen mixing in Venturi devices.
Helium/Oxygen (He/O₂)
Heliox mixture effects on work of breathing.
Oxygen Tank Duration
How long a compressed oxygen cylinder will last.
Liquid Oxygen Duration
How long a liquid oxygen supply will last.
Sizing, Body Metrics & Dosing 7
Body Surface Area (BSA)
Total external body surface from height and weight.
Ideal Body Weight (IBW)
Weight estimate for lung-protective tidal volumes.
ETT Size in Children
Appropriate tube size by age or patient size.
Suction Catheter Size
Appropriate catheter diameter for airway suctioning.
Child vs. Adult Medication
Pediatric dose estimates from an adult dose.
Infant vs. Adult Medication
Infant dose estimates from an adult dose.
Fried's Rule Drug Dose
Age-based infant medication dose estimate.
Clinical Scores & Assessment 5
Apgar Score
Quick newborn condition assessment after birth.
Silverman-Anderson Score
Severity of respiratory distress in newborns.
Glasgow Coma Scale (GCS)
Level of consciousness from eye, verbal, and motor response.
Apnea-Hypopnea Index (AHI)
Events per hour to grade sleep apnea severity.
Smoking Pack-Years
Cumulative tobacco exposure from packs and years.
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These calculators are intended for educational purposes only and are not a substitute for professional clinical judgment, provider guidance, institutional protocols, or verified medical references. Always confirm results before applying them to patient care.
Written by:
John Landry, BS, RRT
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
References
- Haddad M, Sharma S. Physiology, Lung. [Updated 2023 Jul 20]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026.