Travel

Altitude Oxygen Calculator (Air Pressure and Boiling Point)

Enter an elevation to see the air pressure, effective oxygen level and boiling point there.

By TheCalcTools Updated Formula checked and tested

Your numbers

Your results

Air pressure (kPa)
Air pressure (kPa)
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Of sea-level pressure
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Effective oxygen (sea-level equivalent)
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Inspired oxygen pressure (mmHg)
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Boiling point of water (°C)
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Altitude band
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Altitude Oxygen Calculator (Air Pressure and Boiling Point) by TheCalcTools

How the Altitude Oxygen Calculator (Air Pressure and Boiling Point) works

Air pressure falls with height. In the International Standard Atmosphere the pressure up to 11 km is p = 101.325 × (1 − 2.25577 × 10⁻⁵ × h)^5.25588 kPa. Air is always about 20.9% oxygen, but each breath holds fewer oxygen molecules as pressure drops.

The ‘effective oxygen’ figure is the oxygen percentage at sea level that would give the same oxygen pressure, which is how altitude-training tents are rated. Inspired oxygen pressure subtracts water vapour (47 mmHg at body temperature).

Water boils when its vapour pressure equals the air pressure, so it boils at a lower temperature at altitude. Cooking times increase as a result. The boiling point uses the Antoine equation for water.

The formula

p = 101.325 (1 − 2.25577×10⁻⁵ h)^5.25588 kPaEffective O₂ = 20.946% × p ÷ 101.325

Worked example

Using the default values shown in the calculator:

Elevation above sea level (m)3500
  1. At 3,500 m the air pressure is 65.8 kPa, 64.9% of sea level.
  2. That feels like 13.6% oxygen at sea level, and water boils at about 88.4 °C.

Result:

Air pressure (kPa)65.8
Of sea-level pressure64.9%
Effective oxygen (sea-level equivalent)13.6%
Inspired oxygen pressure (mmHg)93
Boiling point of water (°C)88.4
Altitude bandVery high: significant risk; follow acclimatisation rules

Frequently asked questions

Is the oxygen percentage lower at altitude?

No. It stays about 20.9%. The air is thinner, so each breath contains less oxygen.

At what height does altitude sickness start?

Mild symptoms can start around 2,500 m. The usual advice above 3,000 m is to sleep no more than 300 to 500 m higher each night and rest every 3 to 4 days.

Sources and further reading

Hiking Time Calculator (Naismith's Rule)Enter the route distance and total ascent to estimate how long the hike will take Boiling Point Elevation CalculatorEnter molality, the van 't Hoff factor and the solvent to get the boiling point… Pressure ConverterTire pressure, weather and engineering pressure units side by side Dew Point CalculatorThe temperature at which air becomes saturated and dew forms Sunrise and Sunset Calculator (Day Length and Twilight)Enter a date, your coordinates and your UTC offset to get the sunrise and sunset times

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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.