---
title: "Air Pressure & Oxygen at Altitude Calculator | High Nestle"
description: "Air pressure, oxygen partial pressure and equivalent oxygen percentage for any altitude, from the standard atmosphere. Educational, not medical advice."
url: https://highnestle.top/air-pressure-oxygen/
---

# Air pressure & oxygen at altitude

The air up high still holds about 21% oxygen, but there is less air in every breath. See the pressure, the oxygen your lungs actually get, and the sea-level oxygen percentage it equals.

## What the numbers mean

Dry air is 20.95% oxygen at every height you can climb to; what changes is the pressure. The **oxygen partial pressure** is that share of the air pressure. In the lungs the air is warmed to 37 °C and saturated with water vapour (about 47 mmHg, 6.3 kPa), so the **inspired oxygen pressure** is 0.2095 × (pressure − 47 mmHg). Pressure and inspired oxygen fall to about half the sea-level value near 5,500 m and to about 30% on the summit of Everest.

The **equivalent oxygen** figure is the oxygen percentage that would give the same inspired pressure at sea level, the way altitude-training tents describe their settings. Many charts use the simpler pressure ratio, which comes out a little higher, so we show both.

## Standard atmosphere, real weather

Pressures come from the U.S. Standard Atmosphere 1976, an average. Real pressure changes with weather, season and latitude, and is usually a little higher near the equator: on Everest's summit, researchers in 1983 measured 253 mmHg, well above the standard value of about 236 mmHg. If you have a barometer, tick the box and enter its reading. Airliner cabins are pressurised to no more than an 8,000-foot cabin altitude under normal conditions, a US certification rule.

## Not medical advice

Altitude sickness usually starts above about 2,500 m and depends on how fast you climb, not just how high you are. These figures cannot tell you how you will feel. For symptoms, prevention and treatment, read the [NHS guide to altitude sickness](https://www.nhs.uk/conditions/altitude-sickness/) or the [CDC Yellow Book chapter on high-altitude travel](https://www.cdc.gov/yellow-book/hcp/environmental-hazards-risks/high-altitude-travel-and-altitude-illness.html), and ask a doctor before travelling if you have a heart or lung condition.

## Sources

- NOAA, NASA and US Air Force. [U.S. Standard Atmosphere, 1976](https://ntrs.nasa.gov/citations/19770009539). NOAA-S/T 76-1562.
- Peacock AJ. ABC of oxygen: oxygen at high altitude. *BMJ* 1998;317(7165):1063–1066. [doi:10.1136/bmj.317.7165.1063](https://doi.org/10.1136/bmj.317.7165.1063)
- West JB, Lahiri S, Maret KH, Peters RM, Pizzo CJ. Barometric pressures at extreme altitudes on Mt. Everest: physiological significance. *Journal of Applied Physiology* 1983;54(5):1188–1194. [doi:10.1152/jappl.1983.54.5.1188](https://doi.org/10.1152/jappl.1983.54.5.1188)
- US Code of Federal Regulations, 14 CFR 25.841(a). [Pressurized cabins](https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-25/subpart-D/subject-group-ECFRc61d71ee0787390/section-25.841).
- NHS. [Altitude sickness](https://www.nhs.uk/conditions/altitude-sickness/).
- CDC Yellow Book. [High-altitude travel and altitude illness](https://www.cdc.gov/yellow-book/hcp/environmental-hazards-risks/high-altitude-travel-and-altitude-illness.html).

## Questions

**Is there less oxygen in the air at altitude?**

The percentage is the same, about 21%. There is less air, so each breath carries fewer oxygen molecules, which is what the partial pressure measures.

**Why are there two equivalent oxygen figures?**

The simple one scales 20.9% by the pressure ratio. The other first subtracts the water vapour in your lungs, which matters more as pressure falls, so it is a little lower.

**Can this tell me if I will get altitude sickness?**

No. It describes the air, not your body. Ascent rate, fitness and individual susceptibility all play a part; follow medical guidance and descend if symptoms get worse.