Altitude oxygen calculator
A pulse oximeter that read 98% at home will read 90% on the first night in a mountain town, and that is the air, not you. Enter an elevation, or pick a place, to see how much oxygen each breath delivers up there and the blood oxygen range a healthy visitor should expect before acclimatising. Add your own reading to see where it sits.
How the numbers are worked out
Air pressure comes from the International Standard Atmosphere, the model aviation uses for an average day. Real pressure on a given afternoon differs by a few percent with the weather, which is why nothing is quoted past a whole number. The oxygen reaching the lungs is the textbook inspired partial pressure: the 21% of air that is oxygen, applied to what is left of the pressure once the airways have added water vapour at body temperature. That last figure is what actually drives blood oxygen, and it is why the fraction of sea-level oxygen falls slightly faster than the pressure does.
The expected SpO2 is not a formula. Saturation depends on how hard someone is breathing, how long they have been up and where they sit on the oxygen dissociation curve, which is flat near the top and steep below about 90%. The figures here are the central tendency reported for healthy adults from low altitude, at rest, in their first day or two at each height, with a range wide enough to cover the ordinary spread between people.
What changes as you acclimatise
Within hours the body breathes faster and deeper, which blows off carbon dioxide and lifts oxygen a little. Over days the kidneys let the blood turn more acidic to allow that faster breathing to continue, and over weeks red cell production rises. The practical result is that resting SpO2 at a given height climbs by a few points across the first week. A reading that is rising is the reassuring pattern. A reading that is flat or falling, especially with headache, nausea or breathlessness at rest, is the pattern that goes with altitude illness, and the response to that is to stop ascending, not to keep measuring.
Why nights matter more than spot checks
Breathing drive relaxes in sleep, and above roughly 2,500 m most people develop periodic breathing: a repeating cycle of deep breaths, shallow ones and short pauses that shows up on an overnight trace as a regular wave in SpO2. The lowest oxygen of a trip is almost always at night, and a daytime reading cannot see it. Recording a night and loading it into the free session viewer shows the waves directly and gives a number for how much of the night sat below 90%, which is the figure to watch improve. The blog covers what to expect night by night.