Blood Oxygen Saturation (SpO2) Interpreter 🌎

How it works

The calculator classifies your SpO₂ reading according to internationally recognized thresholds from the World Health Organization (WHO), the American Thoracic Society (ATS), and the European Respiratory Society (ERS):

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How SpO₂ is actually measured

Pulse oximeters work by shining two wavelengths of light (red ~660 nm and infrared ~940 nm) through a fingertip or earlobe. Oxygenated hemoglobin and deoxygenated hemoglobin absorb these wavelengths at different ratios. The device calculates the ratio of pulsatile (arterial) to non-pulsatile absorption and converts it to a percentage — the functional oxygen saturation of hemoglobin.

This is distinct from SaO₂ (arterial oxygen saturation measured by blood gas analysis), which is the clinical gold standard. In healthy individuals the two values are very close, but SpO₂ can overestimate true saturation by 2–4 percentage points in people with darker skin tones — a measurement bias documented in peer-reviewed literature including a 2020 study in NEJM — and in conditions such as carbon monoxide poisoning, where carboxyhemoglobin reads as "oxygenated" by standard pulse oximetry.

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Why thresholds matter clinically

The 94% cut-off is not arbitrary. Below this level the oxygen-hemoglobin dissociation curve begins to steepen: a small further drop in SpO₂ causes a disproportionately large drop in the amount of oxygen delivered to tissues. At SpO₂ < 90%, most clinical guidelines — including those from the British Thoracic Society — recommend supplemental oxygen therapy in acute settings. At < 80%, cellular hypoxia and organ damage can develop within minutes.

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Altitude adjustment

At higher elevations the partial pressure of oxygen in the air decreases, so healthy people naturally have lower SpO₂ readings. The calculator shifts the "normal" floor automatically:

For context: Mexico City (~2,240 m) residents routinely have resting SpO₂ values of 92–94%; Cusco, Peru (~3,400 m) averages around 90–92%. Acute mountain sickness symptoms typically appear when SpO₂ drops more than 5 percentage points below an individual's personal sea-level baseline, not just when it crosses a fixed threshold.

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Heart rate context

If you enter your heart rate, the tool flags associated bradycardia (< 60 bpm) or tachycardia (> 100 bpm), both of which can accompany or compensate for low SpO₂. The physiological link is important: when SpO₂ falls, the body's first compensatory response is often to increase heart rate to maintain oxygen delivery (cardiac output × arterial oxygen content). A normal SpO₂ paired with unexplained tachycardia (> 100 bpm at rest) can still warrant evaluation, as it may indicate early respiratory compromise or anemia.

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Common measurement errors

Inaccurate readings are more frequent than most users expect. Key sources of error:

If a reading seems inconsistent with how you feel, rest quietly for 5 minutes, warm your hands, and remeasure.

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What this calculator does NOT include

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Visual output

A colour-coded scale (green → red) shows where your value sits across the full spectrum from critical (< 80%) to normal (≥ 95%).

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> Important: This calculator is an educational reference tool, not a substitute for clinical evaluation. If you are experiencing shortness of breath, chest pain, confusion, or persistent SpO₂ below 94%, seek medical attention promptly.