Why does oxygen saturation fall at altitude?

Barometric pressure drops with elevation, so even though the air is still 21 percent oxygen, the partial pressure of oxygen you breathe is lower. Less driving pressure means less oxygen reaches the blood, so a healthy person's SpO2 falls even though nothing is wrong.

What SpO2 is normal at altitude?

It depends on elevation. At sea level a healthy SpO2 is 96 to 100 percent, but around 2,500 metres a reading near 90 percent is expected and normal. The tool estimates the expected value for the altitude you enter so you can judge whether a reading is concerning.

Which equations does this use?

It estimates local pressure from elevation with the barometric formula, computes inspired oxygen pressure as 0.2095 times pressure minus water vapour, applies the alveolar gas equation for alveolar oxygen, and converts the resulting arterial oxygen to saturation with the Severinghaus oxyhaemoglobin dissociation curve.

Why does the expected carbon dioxide level drop with altitude?

The low oxygen at altitude triggers the hypoxic ventilatory response, so an acclimatised person breathes more and blows off carbon dioxide. The model lowers expected PaCO2 above about 1,500 metres, which raises alveolar oxygen and partly offsets the thinner air.

Can I use this to diagnose altitude illness?

No. These are population estimates for a healthy, acclimatised person and assume a normal heart and lungs. Individual readings vary, acclimatisation takes days, and acute mountain sickness depends on symptoms, not a single number. Treat the output as physiology context, not a clinical threshold.

Oxygen Saturation Altitude Correction Calculator

A pulse oximeter reading that would alarm you at sea level can be completely normal on a mountain. Oxygen saturation falls predictably as you climb, and this calculator estimates what a healthy, acclimatised person should read at any altitude so you can interpret a measurement in context.

How it works

The chain starts with pressure. Barometric pressure falls roughly exponentially with elevation:

Patm = 760 x exp(-elevation_m / 7990)   (mmHg)

That pressure sets the inspired oxygen pressure on room air, after subtracting water vapour at body temperature:

PiO2 = 0.2095 x (Patm - 47)

The alveolar gas equation then gives alveolar oxygen, using a carbon dioxide level that the model lowers at altitude to reflect acclimatised hyperventilation:

PAO2 = PiO2 - PaCO2 / 0.8

Subtracting a normal small alveolar-arterial gradient gives expected arterial oxygen, which the Severinghaus oxyhaemoglobin dissociation curve converts into a saturation percentage.

Reading the result and notes

At altitude a lower SpO2 is the expected normal, not a sign of disease. Near 2,500 metres a healthy reading sits around 90 percent, and it keeps falling higher up. The expected carbon dioxide level drops above about 1,500 metres because the low oxygen drives faster breathing, which partly rescues alveolar oxygen.

These are population estimates for a healthy, acclimatised person with normal heart and lungs. Acclimatisation takes days, individuals vary widely, and altitude illness is diagnosed from symptoms rather than a single saturation value. Use the output as physiology context, not as a clinical cut-off.