SpO2 Oxygen Saturation Guide

Blood oxygen saturation (SpO₂) is the percentage of hemoglobin molecules in the blood that are bound to oxygen. It is measured non-invasively using a pulse oximeter, which shines red and infrared light through a fingertip and measures the differential absorption by oxygenated vs. deoxygenated hemoglobin.

Pulse oximeter measurement principle: SpO₂ (%) = (HbO₂ ÷ (HbO₂ + Hb)) × 100

Where:

• HbO₂: oxyhemoglobin (hemoglobin bound to oxygen)
• Hb: deoxyhemoglobin (hemoglobin not carrying oxygen)

The device calculates the ratio using Beer-Lambert law absorption at two wavelengths:

• Red light (~660 nm): Deoxyhemoglobin absorbs more
• Infrared light (~940 nm): Oxyhemoglobin absorbs more

The R-ratio (internal calculation): R = (AC₆₆₀/DC₆₆₀) ÷ (AC₉₄₀/DC₉₄₀)

The device maps R to SpO₂ via empirically derived calibration curves.

SpO₂ reference ranges at sea level

These are the bands the calculator uses. Each one is measured against the normal floor for your altitude, so at 8,000 ft every band shifts down with it.

Below 80% is life-threatening and sits inside the severe band rather than beside it. The reason the bands are not evenly spaced is the dissociation curve described further down: the drop from 95 to 91 costs far less oxygen delivery than the drop from 90 to 86.

Normal values by context

• Healthy adult at sea level: 96-99%
• Healthy adult at 3,000 m / 10,000 ft: 90-94% is normal, because the partial pressure of oxygen in the air is lower
• During sleep in a healthy person: brief dips to 92-94% are normal
• Sleep apnea: drops to 80-88% during apnea events, which is not normal
• COPD: a baseline of 88-92% may be that person’s acceptable range, and pushing it higher with oxygen can suppress their drive to breathe. This is one to settle with a physician rather than with a chart.

Factors that affect pulse oximeter accuracy:

• Nail polish (especially dark blue/black): can reduce accuracy by 2–3%
• Poor peripheral circulation: cold hands, shock → lower signal quality
• Carbon monoxide poisoning: pulse oximeter reads CO-hemoglobin as oxygenated → falsely high SpO₂ (dangerously misleading, use ABG testing in suspected CO poisoning)
• Skin pigmentation: darker skin tones have shown 3× higher rate of occult hypoxemia missed by pulse oximeters (NEJM 2020 study), be aware of this limitation
• Motion artifact: even slight finger movement causes noise; hold still for accurate reading

Oxygen-hemoglobin dissociation curve: SpO₂ and partial pressure of oxygen (PaO₂) are related non-linearly:

• SpO₂ 98% ≈ PaO₂ 100 mmHg (normal)
• SpO₂ 90% ≈ PaO₂ 60 mmHg (inflection point: below this, PaO₂ drops rapidly)
• SpO₂ 75% ≈ PaO₂ 40 mmHg (mixed venous blood: severely low)

The steep part of that curve, below about 90%, is where a small further drop in saturation costs a large drop in the oxygen actually dissolved in the blood. That is why 90% is the clinical threshold rather than some rounder number, and why the reference bands above get narrower as they fall.