Altitude & Oxygen#

Enter the altitude you’re heading to and this calculator shows the atmospheric pressure there relative to sea level. Since oxygen makes up a constant ~20.9% of dry air, that pressure ratio is also the relative amount of oxygen available per breath compared to sea level.

– of sea-level oxygen

Equivalent to – O₂ at sea level
Pressure – hPa

How it works#

The calculator uses the International Standard Atmosphere (ISA) barometric formula. Up to the tropopause at 11 km, temperature decreases linearly with altitude and pressure follows:

p = p_0 \left(1 - \frac{L h}{T_0}\right)^{\frac{g_0 M}{R L}} \approx p_0 \left(1 - 2.2558 \times 10^{-5} \, h\right)^{5.2559}

where h is the altitude in metres, p₀ = 1013.25 hPa is the sea-level pressure, L = 0.0065 K/m is the standard temperature lapse rate, T₀ = 288.15 K is the sea-level standard temperature, g₀ = 9.807 m/s², M = 0.02897 kg/mol is the molar mass of air, and R = 8.3145 J/(mol·K). Above 11 km the formula switches to the isothermal stratosphere layer. Because the oxygen fraction of air stays essentially constant, the pressure ratio p/p₀ directly gives the relative availability of oxygen.

Reference: U.S. Standard Atmosphere, 1976 (NASA TM X-74335), see also the barometric formula.

Limitations#

This is a standard-atmosphere model. It assumes average conditions and ignores weather. Actual pressure at a given altitude varies by a few percent with weather systems and temperature.
Very high-altitude pressure also varies by latitude and season. Everest summit pressure, for instance, fluctuates roughly between 325 and 343 hPa over the year.
Physiology matters more than the raw number. Acclimatization, fitness and rate of ascent matter far more than pressure ratio alone for how you’ll actually feel.
This is not medical advice. Don’t use it to make safety decisions on the mountain.