Enter any altitude and see the effective oxygen percentage in each breath, plus estimated blood oxygen (SpO₂).
Educational only — always fly your POH & current weatherEstimated SpO₂ vs altitude for an unacclimatized adult. Drag anywhere on the chart to set the altitude.
Educational estimates only — not for medical, mountaineering, or flight-planning use. Individual physiology varies widely.
The air is always about 21% oxygen — but with less pressure pushing it into your lungs, every breath at altitude delivers fewer oxygen molecules. Enter any altitude (or tap Denver, Everest, or a jet cabin) to see the air pressure, the effective oxygen percentage, and an estimated blood oxygen saturation (SpO₂) for an unacclimatized adult. For the full physics-to-physiology chain, drag an aircraft up the atmosphere & hypoxia explorer.
Type an altitude or tap a preset — drag the marker on the chart to explore. SpO₂ estimates are for healthy, unacclimatized adults at acute exposure.
The oxygen fraction of air stays at 20.9% from sea level to the stratosphere. What falls is the air pressure — at 10,000 ft it's about 69% of sea level, at Everest's summit about 31%. Fewer air molecules per breath means proportionally fewer oxygen molecules, so breathing at altitude feels like breathing air with a lower oxygen percentage at sea level. That equivalent number is the effective oxygen percentage this calculator reports.
Your blood doesn't track it linearly. Hemoglobin holds ~90%+ saturation until the oxygen partial pressure drops past a knee in the oxygen–hemoglobin dissociation curve, then falls off fast — that's why you feel roughly fine at 8,000 ft, degraded at 15,000 ft, and lose useful consciousness in minutes at 25,000 ft. Watch that S-curve move in real time in the atmosphere & hypoxia explorer.
| Altitude | Example | Air pressure (% of sea level) | Effective O₂ % | Est. SpO₂ |
|---|---|---|---|---|
| 0 ft | Sea level | 100% | 20.9% | ~98% |
| 5,280 ft | Denver | 82% | 17.2% | ~94% |
| 8,000 ft | Airliner cabin at cruise | 74% | 15.6% | ~90% |
| 10,000 ft | FAA O₂ recommendation | 69% | 14.4% | ~87% |
| 12,500 ft | FAR 91.211 threshold | 62% | 13.1% | ~83% |
| 14,115 ft | Pikes Peak summit | 58% | 12.3% | ~80% |
| 17,598 ft | Everest Base Camp | 51% | 10.6% | ~74% |
| 25,000 ft | "Death zone" begins ~26,000 ft | 37% | 7.8% | ~50% |
| 29,032 ft | Everest summit | 31% | 6.5% | ~24% |
SpO₂ is a model estimate for a healthy, unacclimatized adult at rest on acute exposure — individuals vary by several points, and climbers acclimatize over weeks and run meaningfully higher. Values calibrated to published acute-exposure data (see footer).
| Cabin pressure altitude | FAR 91.211 requirement |
|---|---|
| 12,500 – 14,000 ft | Required flight crew must use O₂ for the portion of flight over 30 minutes |
| Above 14,000 ft | Required flight crew must use O₂ the entire time |
| Above 15,000 ft | Every occupant must be provided supplemental O₂ |
Those are legal minimums, not physiology: night vision measurably degrades above ~5,000 ft and the FAA recommends oxygen above 10,000 ft by day. The full rules, hypoxia types, and time-of-useful-consciousness tables are in our guide to FAR 91.211 oxygen requirements & hypoxia. Flying pressurized? Check what your cabin altitude really is at cruise.
Your body responds to pressure altitude, but your airplane’s performance follows density altitude. See how to calculate density altitude.
The air is still 20.9% oxygen, but at 10,000 ft the pressure is only about 69% of sea level — so each breath delivers about 69% of the oxygen molecules, equivalent to breathing 14.4% oxygen at sea level. The calculator estimates about 87% SpO₂ there for a healthy, unacclimatized adult.
Airliner cabins are pressurized to a cabin altitude of roughly 6,000–8,000 ft at cruise, an effective oxygen percentage of about 15.6–16.8%. That's why a long flight can leave you tired and slightly headachy — the calculator puts your SpO₂ at roughly 90–93% instead of 98%.
Under FAR 91.211, required crew use oxygen above 12,500 ft cabin pressure altitude when that portion of the flight exceeds 30 minutes, continuously above 14,000 ft, and every occupant must be provided oxygen above 15,000 ft. The FAA additionally recommends it above 10,000 ft by day and 5,000 ft at night.
At 29,032 ft the pressure is only about 31% of sea level — an effective oxygen percentage near 6.5%. An unacclimatized person would lose useful consciousness in one to two minutes; climbers survive through weeks of acclimatization, and most still use bottled oxygen.
Readings in the high 80s are common for visitors above ~10,000 ft and usually improve over days of acclimatization. But numbers that would be alarming at sea level still warrant attention at altitude — descend and seek medical advice for symptoms like severe headache, ataxia, or breathlessness at rest. This tool is educational, not medical guidance.
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