What formula does this calculator use?

It uses the International Standard Atmosphere (ISA) barometric formula, which models the atmosphere in layers. Below 11 km (troposphere) pressure follows a power-law relationship with temperature lapse rate; above 11 km (lower stratosphere) it follows an exponential decay at constant temperature.

Why does oxygen percentage stay at roughly 20.9%?

The proportion of oxygen in dry air (20.946%) stays nearly constant up to about 80 km due to atmospheric mixing. What changes is the partial pressure of oxygen — there are fewer oxygen molecules per breath at altitude, even though the percentage mix is the same.

What is the effective oxygen percentage shown?

It represents the equivalent oxygen concentration at sea-level pressure that would deliver the same amount of oxygen per breath. For example, at 5,500 m (18,000 ft) the air pressure is roughly half of sea level, so breathing feels like inhaling 10.5% oxygen at sea level.

How accurate are the results?

Results match the ISA model closely for standard day conditions. Real-world pressure varies with weather, temperature, and humidity. For precise aviation use, always consult certified instruments and official meteorological data.

What is the highest altitude supported?

The calculator supports altitudes up to 86,000 m (86 km), covering the troposphere, tropopause, and most of the stratosphere. Beyond that, the ISA model requires additional layers.

Can I use feet instead of meters?

Yes. Toggle the unit selector to feet and enter your altitude. The calculator converts internally before applying the formula.

Altitude Air Pressure Calculator

Enter an altitude to instantly calculate air pressure, air density, and effective oxygen percentage using the International Standard Atmosphere model.

Enter Altitude

Sea level (0) Everest (8,848 m)

Famous Altitudes

Atmospheric Conditions

Air Pressure

—

hPa (mbar)

— inHg

— psi

Temperature (ISA)

—

°C

— °F

Air Density

—

kg/m³

Effective Oxygen

—

% (sea-level equivalent)

vs. Sea Level

Air Pressure —%

Air Density —%

Effective Oxygen —%

Altitude Advisory

—

Summary

Enter an altitude to instantly calculate air pressure, air density, and effective oxygen percentage using the International Standard Atmosphere model.

How it works

Enter an altitude in meters or feet using the unit toggle.
The calculator applies the ISA barometric formula for the appropriate atmospheric layer.
Air pressure, air density, temperature, and oxygen percentage are computed instantly.
Compare results against sea-level baselines shown as percentage bars.
Use the quick-select buttons for famous landmark altitudes.

Use cases

Hikers and mountaineers estimating oxygen availability at altitude.
Pilots cross-checking pressure altitude for flight planning.
Travelers preparing for high-altitude destinations like Cusco or Lhasa.
Athletes calibrating training loads for high-altitude camps.
Engineers sizing pressurization or HVAC systems for elevated installations.
Educators demonstrating atmospheric physics in the classroom.
Divers understanding surface pressure changes affecting decompression.
Paragliders and skydivers assessing air density for glide calculations.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu