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The air at altitude hasn't changed. It's still 20.9% oxygen — the same mix you breathe at sea level. What changes is pressure. Every meter you climb, the atmospheric column above you shrinks, and so does the force pushing oxygen molecules into your lungs. At the summit of Mont Blanc (4,808m), you're inhaling 58% of the oxygen molecules per breath that you would at sea level. On Everest (8,849m), that figure drops to 31%.
That gap is what separates a comfortable trail run from a life-threatening summit push. And it's why understanding exactly how oxygen levels change with altitude — not in vague terms, but in actual numbers — matters so much.
Use the Oxymeter calculator to get a precise figure for any elevation before you head out.
Why oxygen decreases with altitude: the simple version
The ICAO Standard Atmosphere gives us a precise formula for how oxygen availability (measured as a fraction of sea level) falls with altitude. The relationship isn't linear — it's a power curve that gets steeper the higher you go.
At 2,000 metres, you've lost about 22% of sea-level oxygen. At 5,000 metres, you've lost nearly half. At the top of Everest, you're running on less than a third. The body can adapt — but only so far, and only slowly.
Altitude sickness, reduced aerobic capacity, impaired decision-making: these aren't signs of weakness. They're the predictable result of physiology working against arithmetic.
Altitude and oxygen levels: complete chart
This table uses the ICAO Standard Atmosphere formula. SpO₂ ranges are typical resting values for a healthy, non-acclimatized adult. Acclimatized climbers often read 2–4 points higher.
| Altitude | Feet | O₂ vs sea level | Typical resting SpO₂ | What to expect |
|---|---|---|---|---|
| 0 m | 0 ft | 100% | 98–99% | Normal breathing |
| 500 m | 1,640 ft | 94% | 97–99% | No symptoms |
| 1,000 m | 3,281 ft | 89% | 96–98% | No symptoms at rest |
| 1,500 m | 4,921 ft | 83% | 95–97% | Slightly faster breathing during effort |
| 2,000 m | 6,562 ft | 78% | 94–96% | Breathlessness on hard exertion |
| 2,500 m | 8,202 ft | 74% | 92–95% | Shortness of breath; headache possible |
| 3,000 m | 9,843 ft | 69% | 90–93% | AMS risk starts here |
| 3,500 m | 11,483 ft | 65% | 88–91% | Significant AMS risk; slower recovery |
| 4,000 m | 13,123 ft | 61% | 85–89% | Acclimatization required |
| 4,500 m | 14,764 ft | 57% | 82–87% | HACE/HAPE risk increases sharply |
| 5,000 m | 16,404 ft | 53% | 79–84% | Supplemental oxygen often needed |
| 5,895 m | 19,341 ft | 47% | 73–78% | Kilimanjaro summit |
| 6,000 m | 19,685 ft | 47% | 72–77% | Extreme altitude |
| 7,000 m | 22,966 ft | 41% | 66–71% | Death zone approaching |
| 8,000 m | 26,247 ft | 35% | 60–65% | Death zone |
| 8,849 m | 29,032 ft | 31% | 56–61% | Everest summit |
SpO₂ ranges are approximations. Individual variation is significant — fitness, genetics, acclimatization status, and rate of ascent all affect where you land within these bands.
How to know your exact altitude
The chart above is only useful if you know where you actually are. Phone map apps are notoriously vague about elevation — they're optimized for horizontal position, not vertical, and can be off by 50–100 metres. There are two reliable ways to pin it down.
Free — the Oxymeter tool. Open the Oxymeter calculator, let it read your GPS position, and it returns your elevation along with the expected oxygen level and SpO₂ for that exact spot. It's a Progressive Web App: install it on your phone straight from the browser menu and it keeps working offline — useful above the treeline where there's no signal but you still want to read this chart against your real altitude.
Precise and offline — a barometric altimeter watch. Serious climbers don't trust phone GPS for elevation. A watch with a barometric altimeter (the "ABC" sensor set: altimeter-barometer-compass) reads height from air pressure, works with no signal, and tracks your ascent rate — the single most useful number for pacing acclimatization. The current references worth knowing:
- Garmin Instinct 2 — rugged, weeks of battery life, the value benchmark for ABC watches
- Garmin Fenix 8 — full topo mapping plus barometric altimeter and Pulse Ox in one device
- Suunto 9 Peak — lightweight, strong barometric accuracy, a favourite in the Alps
- Coros Apex 2 Pro — exceptional battery life for multi-week expeditions
A dedicated altimeter watch also reads SpO₂ at the wrist, but for the decisions that matter a fingertip reading is more reliable — see the section below.
What is normal SpO₂ at altitude?
This is the question most hikers have, and the honest answer is: it depends where you are.
The numbers that matter aren't absolute — they're relative to elevation. An SpO₂ of 92% would send a doctor reaching for supplemental oxygen in a hospital. On a three-night acclimatization at 3,500 metres, it's perfectly expected.
Here's a practical benchmark table:
| Elevation | Expected resting SpO₂ | Action if below |
|---|---|---|
| Below 2,000 m | ≥ 95% | Monitor |
| 2,000–3,000 m | 90–96% | Rest; monitor |
| 3,000–4,000 m | 87–93% | Slow ascent; rest day if < 85% |
| 4,000–5,000 m | 82–89% | Rest day; consider descent if < 80% |
| Above 5,000 m | 75–85% | Expert-guided decisions only |
A pulse oximeter gives you these numbers in real time, without guessing. That's exactly why climbers carry them.
Warning signs: what SpO₂ values are alarming at altitude
The raw number matters less than the trend. A steady 88% at 3,500m after a rest day is reassuring. A 88% that was 93% this morning — and you haven't climbed — is a problem.
Absolute thresholds to take seriously:
- Below 90% at rest at any altitude — stop ascending; take a rest day
- Below 85% at rest — strong signal to descend; do not continue up
- Below 80% at rest — descend immediately; this is an emergency at any elevation
SpO₂ values during exercise will always be lower. Don't measure while moving. Sit quietly for three minutes first — let your heart rate settle — then take the reading.
Symptoms matter too. Headache, nausea, loss of appetite, confusion: these are the body's language for "something is wrong." A pulse oximeter confirms it. But don't wait for a perfect number to act on obvious symptoms.
How to monitor your SpO₂ at altitude
You can't manage what you can't measure. A fingertip pulse oximeter weighs under 50 grams and costs less than a decent pair of hiking socks. For any trip above 3,000 metres, it's not optional equipment — it's how you make objective decisions when your brain may already be slightly impaired by hypoxia.
Take your SpO₂ every morning before getting out of your sleeping bag. Note the number. Compare it to the previous day. If it's dropping on a rest day, that's data. If it's recovering after acclimatization, that's also data.
To find the right device for your style of adventure — from day hikes to expedition climbing — see our guide to the best pulse oximeters for hiking.
Frequently Asked Questions
What is normal SpO2 at altitude?
Normal SpO₂ at altitude depends entirely on the elevation. At sea level it's 98–99%. At 2,000m (6,562ft) it drops to 94–96% at rest. At 3,500m (11,483ft), 88–91% is typical. At 5,000m, 79–84% is expected. Readings that look alarming at sea level are perfectly normal higher up — context is everything.
What is a normal oxygen level at high altitude?
Above 3,000m, a normal resting SpO₂ for a healthy, acclimatized person is 88–93%. Below 3,500m, most adults stay above 90% at rest. Values below 85% at rest are a warning sign at any elevation — it's not the altitude that makes it acceptable, it's the trend.
How much less oxygen is there at 7000 feet?
At 7,000 feet (2,134m), the partial pressure of oxygen is about 77% of sea level. Your SpO₂ will typically read 93–96% at rest. You'll feel the difference during hard uphill effort — your legs will be fine, your lungs will be working harder than expected.
How much less oxygen is there at 8000 feet?
At 8,000 feet (2,438m), oxygen availability drops to roughly 74% of sea level. Most fit hikers handle this well, especially after a day or two of acclimatization. Expect breathlessness on steep sections and slower recovery between efforts.
How much oxygen is at 40000 feet?
At 40,000 feet (12,192m) — commercial cruising altitude — oxygen availability is just 18% of sea level. Without a pressurised cabin, you'd become incapacitated within minutes. Commercial aircraft maintain cabin pressure equivalent to roughly 6,000–8,000 feet (1,800–2,400m), keeping passengers comfortable at 93–97% SpO₂.
What is an unsafe pulse ox?
At sea level, SpO₂ below 94% is considered low and warrants medical evaluation. At altitude, thresholds shift downward — but below 85% at rest is universally a warning sign, regardless of elevation. Below 80% at altitude: descend. Don't wait for symptoms to worsen.
What is an alarming oxygen level?
At altitude, an SpO₂ below 80% at rest is alarming and typically indicates the need to descend immediately. Between 80–85% at rest — especially if dropping — monitor closely and strongly consider descending. Symptoms always override numbers: if you feel seriously unwell, descend regardless of what the oximeter reads.
Is an oxygen level of 94 good to fly?
It's borderline. Aircraft cabins are pressurised to roughly 1,800–2,400m equivalent altitude, which can push your SpO₂ down by 2–4 percentage points. If you're at 94% on the ground, you may land at 90–91% in flight — uncomfortable, and potentially problematic if you have a respiratory or cardiovascular condition. Consult your doctor before flying if your baseline SpO₂ is below 95%.
For a deeper look at how altitude affects the body and what to do when SpO₂ drops, read our guide to oxygen saturation at high altitude and altitude sickness.
