Blood Oxygen Level Guide
Look up what any SpO2 reading means from a pulse oximeter.
Explains normal (95-100%), low (90-94%), and critically low levels with symptoms and guidance.
Blood oxygen level (SpO2) measures the percentage of hemoglobin molecules in your blood that are carrying oxygen. Pulse oximeters estimate this non-invasively using light absorption at two wavelengths (red ~660 nm and infrared ~940 nm).
The underlying formula (Beer-Lambert Law basis):
SpO2 = (Ratio of oxygenated Hb / Total Hb) × 100
In practice, pulse oximeters use a calibration curve derived from this ratio (R):
R = (AC660 / DC660) / (AC940 / DC940)
Where AC = pulsatile (arterial) signal and DC = baseline signal at each wavelength. SpO2 is then looked up from a stored R-curve. You never calculate this by hand, since the oximeter does it in the clip on your finger.
What the numbers mean:
| SpO2 Range | Interpretation |
|---|---|
| 95–100% | Normal, healthy oxygenation |
| 90–94% | Below normal. Mild hypoxemia, worth monitoring |
| 85–89% | Low. Moderate hypoxemia, seek medical attention |
| Below 85% | Severe hypoxemia. Emergency |
Altitude changes what is normal, not what is dangerous
This distinction is the whole reason this calculator reports two things rather than one.
At 8,000 ft your baseline saturation genuinely sits a few points lower than it would at sea level, so a reading of 93% there is unremarkable where at sea level it would prompt a second look. That is a statement about what is expected.
It is not a statement about what is safe. Saturation of 84% means 84% of your hemoglobin is carrying oxygen, and your tissues do not care why. Some calculators add a few points for altitude and then judge the adjusted figure against sea-level danger thresholds, which quietly turns an emergency reading into a “seek medical attention” one. This page does not do that: the clinical grade always comes from the raw number, and the altitude comparison is reported alongside it.
Typical baselines by altitude:
| Altitude | Expected resting SpO2 | Typical drop |
|---|---|---|
| Sea level, under 2,000 ft | 97–99% | none |
| 2,000–5,000 ft | 96–98% | about 1 point |
| 5,000–8,000 ft | 94–96% | about 3 points |
| 8,000–10,000 ft | 92–94% | about 5 points |
Other normal variation:
- During deep sleep, SpO2 may briefly dip to 90–92% even in healthy people
- Smokers and people with COPD often have lower baseline readings, and their target range may deliberately be lower
Accuracy notes:
- Nail polish (especially dark colors) and acrylic nails can cause falsely low readings: use a fingertip without polish or try the earlobe
- Poor circulation (cold hands) reduces accuracy significantly: warm up before measuring
- Carbon monoxide poisoning produces falsely normal SpO2 readings: oximeters cannot distinguish COHb from O2Hb
- For clinical accuracy, arterial blood gas (ABG) testing is the gold standard
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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