From your location, or any altitude you choose.
Check your oxygen availability and altitude now to move safely at high altitudes.
The fraction of oxygen in air never changes. On the summit of Everest it is still 20.9%, exactly what it is on a beach. What collapses as you climb is air pressure, and pressure is what pushes oxygen across the lining of your lungs into your blood. That is what the model tracks.
Enter an altitude and the engine applies the International Standard Atmosphere (ISA, ISO 2533:1975 — the model aviation has run on since the 1950s, published as ICAO Doc 7488): pressure at that altitude, the partial pressure of oxygen inside it, then the ratio against sea level. One number comes out, and that number is honest.
At 2,000 m each breath delivers 78% of the oxygen you get at sea level. At 3,000 m, 69%. At 4,000 m — Mont Blanc, the Colorado 14ers, the high summer huts — 61%. Look at the slope. Between 3,000 and 4,000 metres you shed eight points over a thousand metres of ascent, and that band is where ordinary people run into trouble. Not on Everest.
The figure is oxygen available in the air, not your saturation. Two different quantities, and the calculator does not estimate the second one.
For a sense of scale: at 3,000 m a healthy adult at rest, in the first days, typically reads an SpO₂ between 88 and 93%; after a week of adaptation that climbs to 92-96%. At 4,000 m, unacclimatized, 82-88%. These are population bands drawn from resting measurements with a fingertip oximeter. Your actual value comes from the device on your finger, up there.
It is not a diagnosis either. AMS risk is not written in the altitude: it follows how fast you climb, how many nights you have already slept high, and how your body behaved last time. Two people on the same col, the same percentage, opposite outcomes. It happens constantly.
For the full series altitude by altitude, see the oxygen levels at altitude chart. To know what your pulse oximeter should read and when a number is worth worrying about, read oxygen saturation at altitude. Planning a real ascent and want to act before the headache arrives? Start with altitude sickness.
The International Standard Atmosphere (ISA, ISO 2533:1975) — the same model civil aviation calibrates its instruments against, also published as ICAO Doc 7488. It derives pressure from altitude, the partial pressure of oxygen from pressure, and compares that with sea level. The model describes an average atmosphere: a real day can sit a few hPa off, and with it a percentage point.
No, and no atmospheric model can. The calculator tells you how much oxygen the air holds at that altitude. How much of it reaches your blood depends on you — fitness, acclimatization, hydration, genetics — and only a pulse oximeter on your finger measures that.
There is no threshold altitude. AMS usually shows up above 2,500 m, but what drives it is ascent rate, the altitude you sleep at, and how your body behaved last time: people who have been sick before tend to get sick again. A low oxygen figure flags the load, not the diagnosis.
No. The altitude field comes first and works on its own: type the metres of your target and calculate. GPS is a shortcut for people already on the mountain, and vertical position is what phones get wrong most — 50 to 100 metres off is routine.