Oxygen Delivery & Respiratory Support Calculator

Introduction to oxygen delivery and respiratory support

Oxygen therapy treats hypoxemia, but the number printed beside a device is only part of the clinical picture. A nasal cannula, simple mask, reservoir mask, high-flow nasal cannula, CPAP system, and mechanical ventilator deliver oxygen in different ways. Low-flow devices allow the patient to entrain room air, so their effective oxygen concentration changes with inspiratory demand. Blended systems instead deliver a selected fraction of inspired oxygen, usually written as FiO₂.

This educational calculator estimates FiO₂ for the selected device and then relates oxygen saturation to the amount of support being provided. It calculates the SpO₂/FiO₂ ratio, an estimated PaO₂/FiO₂ ratio, and the ROX index used when following patients on high-flow nasal cannula. Optional arterial blood gas, hemoglobin, altitude, and cylinder inputs add the alveolar gas equation, A–a gradient, arterial oxygen content, and approximate portable-cylinder runtime.

These outputs organize information; they do not prescribe oxygen. Delivered FiO₂ can be lower than expected because of rapid breathing, mouth breathing, a loose mask, a collapsed reservoir bag, excessive leaks, or an oxygen source that is not supplying the intended flow. A clinician must also assess work of breathing, mental status, perfusion, carbon-dioxide retention, hemodynamics, and the cause of respiratory failure.

Evidence context: ROX thresholds are commonly traced to Roca and colleagues’ high-flow nasal cannula studies. The S/F-to-P/F relationship follows published surrogate models such as Rice and colleagues. Oxygen targets should follow current local guidance. This page was reviewed July 10, 2026.

How to use the oxygen and FiO₂ calculator

Begin by selecting the patient description and oxygen device. For room air, the calculator uses 21%. For a low-flow nasal cannula, simple face mask, or non-rebreather mask, enter the flow in litres per minute. For HFNC, CPAP, or a ventilator, enter the FiO₂ displayed by the blender or machine. Flow remains useful for estimating oxygen consumption, but it does not determine the blended concentration.

Next enter the current SpO₂ and respiratory rate, then choose the intended saturation range. The selected target is used for comparison, not as a recommendation. Open the advanced inputs when an arterial blood gas or hemoglobin value is available, when altitude materially differs from sea level, or when a portable cylinder’s duration matters. Select Calculate Oxygen Support to generate the summary. The copy and CSV controls become available only after a valid calculation.

A pulse-oximeter value should be checked for a stable waveform and plausible pulse rate. If a displayed saturation conflicts with the patient’s appearance, repeat the measurement at another site and consider arterial blood gas analysis. Escalation must never be delayed merely because one calculated ratio appears reassuring.

Formulas and assumptions used for delivered FiO₂

The low-flow nasal cannula estimate uses the familiar teaching approximation below. It is most useful from 1 to 6 L/min and should not be treated as a direct measurement.

At 3 L/min, this rule gives approximately 33% FiO₂. A patient whose inspiratory flow exceeds the cannula flow draws extra room air around the prongs, reducing the actual concentration. The simple-mask model interpolates through a broad 35–55% teaching range at 6–10 L/min. A simple mask generally needs at least 5–6 L/min to reduce rebreathing of exhaled gas. The non-rebreather model rises from about 60% at 10 L/min toward 90% at 15 L/min, assuming a good seal and a reservoir that remains inflated during inspiration.

For the alveolar calculation, FiO₂ is converted from a percentage to a fraction. Barometric pressure is estimated from altitude, water-vapor pressure is taken as 47 mmHg at body temperature, and the respiratory quotient is assumed to be 0.8.

The A–a gradient is alveolar PAO₂ minus measured arterial PaO₂. A larger gradient can reflect ventilation-perfusion mismatch, shunt, or impaired diffusion, but interpretation depends on age, FiO₂, pressure conditions, and clinical setting. A negative result usually means the entered FiO₂, PaCO₂, altitude, or blood gas values are inconsistent.

Interpreting ROX, S/F, P/F, and oxygen content

The S/F ratio divides SpO₂ by FiO₂ expressed as a fraction. It offers a non-invasive oxygenation trend when arterial PaO₂ is unavailable. Published approximations place an S/F near 315 around a P/F of 300 and an S/F near 235 around a P/F of 200. The relationship becomes unreliable when SpO₂ is above roughly 96–97%, because saturation reaches the flat part of the oxyhemoglobin dissociation curve.

The estimated P/F value uses S/F ≈ 64 + 0.84 × P/F. If arterial PaO₂ is entered, the calculator replaces that estimate with measured PaO₂ divided by FiO₂. P/F ranges often associated with mild, moderate, and severe ARDS are meaningful only when the remaining diagnostic criteria and required pressure support are present. A ratio alone does not diagnose ARDS.

The ROX index is S/F divided by respiratory rate. In frequently cited HFNC cohorts, values at or above 4.88 were associated with a lower likelihood of intubation, while values below 3.85 raised concern for failure. Timing and trend matter. The calculator therefore displays ROX only when HFNC is selected and does not present it as an automatic escalation rule.

Arterial oxygen content combines oxygen bound to hemoglobin with a much smaller dissolved component. A patient with severe anemia may have a normal saturation but reduced oxygen content, which illustrates why SpO₂ alone does not describe total oxygen delivery to tissues.

Worked example: pneumonia treated with nasal cannula

Consider a 65-year-old adult with pneumonia, an SpO₂ of 88%, and a respiratory rate of 24 breaths per minute. A nasal cannula is started at 4 L/min. The low-flow rule estimates FiO₂ as 21% + (4% × 4), or 37%. The S/F ratio is 88 ÷ 0.37, approximately 238. Applying the surrogate relationship gives an estimated P/F close to 207.

Those numbers suggest materially impaired oxygenation, but they do not identify the cause or dictate the next device. Reassessment should confirm the oximeter signal and cannula position, evaluate work of breathing, and check whether SpO₂ enters the prescribed range. Persistent tachypnea, exhaustion, altered mental status, instability, or worsening gas exchange requires urgent clinical review even when saturation improves.

If the same patient had COPD with a known risk of hypercapnic respiratory failure, a clinician might prescribe a lower target such as 88–92%. Giving enough oxygen to drive saturation far beyond that range can worsen CO₂ retention in susceptible patients. This is why the target input must represent an actual clinical plan rather than a default chosen without context.

Typical oxygen-device flows and concentration ranges

The ranges below are broad educational values. Equipment specifications and local protocols take priority, and actual performance varies with fit, leaks, breathing pattern, and total inspiratory demand.

SpO₂ targets and bedside context

Common educational ranges are 94–98% for many acutely ill adults and 88–92% for patients at risk of hypercapnic respiratory failure. These are not universal targets. Pregnancy, congenital heart disease, neonatal care, carbon-monoxide exposure, palliative goals, and other circumstances require specific guidance. Hyperoxia can be harmful, so oxygen should be titrated rather than continued automatically at the highest setting.

FiO₂ describes the oxygen concentration being inhaled, while SpO₂ estimates the proportion of hemoglobin binding sites occupied by oxygen. Neither measures ventilation. A patient can retain dangerous amounts of CO₂ while maintaining an acceptable saturation. Oximetry can also be affected by motion, poor perfusion, skin pigmentation, nail products, venous pulsation, ambient light, and dyshemoglobinemia.

Escalation, monitoring, and safety limitations

Before escalating, check the oxygen source, tubing, connections, device fit, selected flow, and pulse-oximeter waveform. Observe the patient for accessory-muscle use, inability to speak, diaphoresis, agitation, somnolence, cyanosis, or exhaustion. Treating pneumonia, edema, bronchospasm, pulmonary embolism, sepsis, or another cause is as important as selecting a device.

This calculator does not model PEEP, mean airway pressure, lung compliance, dead space, shunt fraction, cardiac output, tissue oxygen extraction, or the full oxyhemoglobin dissociation curve. Its FiO₂ values are estimates, its S/F-derived P/F value is a surrogate, and its cylinder runtime excludes reserve policies, regulator variation, leaks, and unusable residual pressure. Use a safety reserve when planning transport.

All results are educational. Persistent hypoxemia, altered mental status, severe work of breathing, hemodynamic instability, or suspected respiratory arrest requires immediate professional assessment. Never delay emergency assistance while entering values into a calculator.

Frequently asked questions about oxygen delivery