Why is water's boiling point lower at high altitudes?

Boiling occurs when the vapor pressure of a liquid is equal to the air pressure surrounding it. At higher altitudes, the air pressure is lower, so less heat is required to reach the boiling point and water boils at a lower temperature. At an altitude of about 1200 meters (3940 feet), the boiling point of water is already close to 96 °C instead of 100 °C.

How to calculate air pressure at a given altitude?

First the standard sea level pressure and temperature are established, then a formula is used to calculate air pressure in the troposphere. The sea level pressure is multiplied by the gradient and altitude, divided by surface temperature, plus one factor. This sum is then raised to an exponent which consists of gravity, molar mass of air, and gas constant.

What is the air pressure at the top of Mount Everest?

At an altitude of about 8,849 m the standard air pressure is about 314 hPa which is approximately 0.31 atm and less than one third of the value at sea level. For this reason nearly all climbers carry extra oxygen with them when climbing Mount Everest.

Why is cabin pressure regulation necessary on passenger aircraft?

Passenger planes usually fly at an altitude of about 11 km. At this height, the air pressure is only about one fifth of that at sea level, which is too low to breathe comfortably. The cabin is pressurized to a pressure roughly equivalent to being at an altitude of between 6000 and 8000 feet (1800-2400 m). This is a compromise between passenger comfort and the structural integrity of the plane.

Does air pressure change at high altitudes depending on temperature?

Yes. As warmer air layers have a lower density, the atmospheric pressure at the same altitude can be different depending on weather conditions. The tool allows you to set the sea temperature and does not assume a fixed value.

Air Pressure at Altitude Calculator

Air pressure at altitude

What do you want to find?

Pressure at altitude

Altitude from pressure

Altitude

Air pressure at sea level

Air temperature at sea level

Air pressure at altitude (hPa)

Air pressure at altitude

hPa

Percent of sea-level pressure: %
Air temperature at that altitude: °C

Water boils at: °C
Effective oxygen: %

At 0 m, the air pressure is about 1013.25 hPa — 100% of your sea-level value.

At this height most people feel no effect from the reduced pressure.

Chart & reference table

Show the pressure vs. altitude chart

Plot how pressure falls as you climb.

Show the altitude reference table

A quick lookup from sea level up to 11 km.

Standard-atmosphere pressure by altitude
Altitude (m)	Pressure (hPa)	Percent of sea level	Water boils (°C)
0	1,013.25	100	100
500	954.61	94.2	98.3
1,000	898.75	88.7	96.7
1,500	845.56	83.5	95
2,000	794.95	78.5	93.4
3,000	701.09	69.2	90
5,000	540.2	53.3	83.3
8,000	356	35.1	73.2
11,000	226.32	22.3	62.8

Air pressure decreases as altitude increases. This tool uses a formula to calculate air pressure in the International Standard Atmosphere and calculates air pressure for any given altitude.

It supports calculations in both directions. You can either enter the altitude to find out what the corresponding air pressure is or you can enter a measured air pressure and it will tell you the corresponding altitude.

In addition to the air pressure, it also displays the relative air pressure compared with sea level, the temperature at that altitude, the boiling point of water and the actual amount of oxygen present in the breathing air.

The reason why air pressure decreases with increasing altitude is as follows:

Atmospheric pressure at a particular location is the weight of all the air above that point. The higher you go, the less mass of air there is above you, resulting in lower atmospheric pressure.

The decrease in air pressure is not a straight line. Because the air is compressible, the lowest layer of air is compressed more, making it denser and carrying most of the weight of the air above it.

In regions close to the ground, air pressure is halved for every increase of about 5.5 km in altitude. Even at the border with the troposphere, i.e. at an altitude of around 11 km, the air above makes up only about one fifth of the entire atmosphere.

Instructions for using the tool:

Select desired area. In the Calculate Altitude mode enter altitude, in the Calculate Pressure mode enter air pressure.

The two standard fields, namely sea level pressure and temperature, use the default ISA values of 1013.25 hPa and 15 °C respectively. If you want to estimate values under non-standard atmospheric conditions, then you can keep these values as they are. But you can also adjust them to match actual weather reports.

Each field has a unit toggle so you can use either hPa, kPa, atm, psi or inches of mercury for the air pressure and meters or feet for length.

Formula for calculating air pressure:

In the troposphere, temperature decreases at a constant rate with increasing altitude and air pressure follows this relationship.

P = P_0\left(1 + \frac{L\,h}{T_0}\right)^{\!-\frac{g M}{R L}}

where P0 and T0 are the pressure and temperature at sea level respectively, h is altitude and L is the temperature gradient of -0.0065 K/m. The exponent consists of the following constants and the result of the calculation is approximately 5.2559.

Symbol	Meaning	Value
P0	Pressure at sea level	101,325 Pa (1 atm)
T0	Temperature at sea level	288.15 K (15 °C)
L	Temperature lapse rate	-0.0065 K/m
g	Gravitational acceleration	9.80665 m/s²
M	Molar mass of air	0.0289644 kg/mol
R	Universal gas constant	8.31432 N·m/(mol·K)

Above the tropopause, temperature remains constant so the tool switches to an isothermal mode and air pressure decreases exponentially with altitude.

For example, if the altitude is 5,000 meters and conditions at sea level are standard, then the result from the formula will be about 540 hPa, which is roughly 0.53 atm, just over half of the air pressure at sea level.

Fields of application:

Pilots and skydivers use the relationship between air pressure and altitude. An altimeter is actually a barometer that measures altitude using air pressure.

Hikers and mountaineers use this information to predict how thin the air will be and to plan acclimatization. Engineers use it to assess equipment that must operate at high altitudes.

Chefs also face this problem in their work as lower air pressure changes the time it takes to cook or bake.

Boiling point, oxygen, death zone.

Water begins to boil when its vapor pressure is equal to the surrounding air pressure. The lower the air pressure, the lower the boiling point. So at an altitude of 3,000 meters, water boils at about 90 °C rather than 100 °C. Therefore, cooking food takes longer.

Regardless of the altitude, the air still contains 20.9 percent oxygen, but because the entire atmosphere is thinner, each breath takes in fewer molecules. The term "effective oxygen content" gives this concentration as being equivalent to sea level.

At the summit of Mount Everest, air pressure is only about one-third that at sea level. Areas above 8,000 meters are referred to as the death zone where the human body begins to fail without supplemental oxygen.

This tool is based on an international standard atmosphere and is suitable for general educational purposes and planning. Please note that actual results are estimates based on the standard atmosphere as actual air pressure varies with weather conditions, humidity, and local terrain.

Frequently asked questions

Why is water's boiling point lower at high altitudes?: Boiling occurs when the vapor pressure of a liquid is equal to the air pressure surrounding it. At higher altitudes, the air pressure is lower, so less heat is required to reach the boiling point and water boils at a lower temperature. At an altitude of about 1200 meters (3940 feet), the boiling point of water is already close to 96 °C instead of 100 °C.
How to calculate air pressure at a given altitude?: First the standard sea level pressure and temperature are established, then a formula is used to calculate air pressure in the troposphere. The sea level pressure is multiplied by the gradient and altitude, divided by surface temperature, plus one factor. This sum is then raised to an exponent which consists of gravity, molar mass of air, and gas constant.
What is the air pressure at the top of Mount Everest?: At an altitude of about 8,849 m the standard air pressure is about 314 hPa which is approximately 0.31 atm and less than one third of the value at sea level. For this reason nearly all climbers carry extra oxygen with them when climbing Mount Everest.
Why is cabin pressure regulation necessary on passenger aircraft?: Passenger planes usually fly at an altitude of about 11 km. At this height, the air pressure is only about one fifth of that at sea level, which is too low to breathe comfortably. The cabin is pressurized to a pressure roughly equivalent to being at an altitude of between 6000 and 8000 feet (1800-2400 m). This is a compromise between passenger comfort and the structural integrity of the plane.
Does air pressure change at high altitudes depending on temperature?: Yes. As warmer air layers have a lower density, the atmospheric pressure at the same altitude can be different depending on weather conditions. The tool allows you to set the sea temperature and does not assume a fixed value.

Related calculators

Boiling Point at Altitude CalculatorEnter an altitude, an air pressure, or a target boiling point and the calculator fills in the other two. Solves for water by default, or any liquid in custom mode.

Air Density CalculatorCalculate air density from temperature, pressure or altitude, and humidity — plus density altitude, relative air density, partial pressures, and specific weight.

Pressure CalculatorCalculate pressure from force and contact area with P = F / A, and solve for any of the three. Convert between gauge and absolute pressure, read the result in kPa, bar, psi, atm, and mmHg, and compare it against familiar pressures.

Power Required at Altitude CalculatorFind the power an aircraft needs for level flight at any altitude from the aerodynamic power equation. Solve for power, air density, true airspeed, area, or drag coefficient, with an ISA density helper and a constant-lift climb projection.

BMEP Calculator (Brake Mean Effective Pressure)Work out an engine's brake mean effective pressure from its displacement and torque, in bar and psi, or solve backwards for the torque or displacement a target BMEP would need.

Air-Fuel Ratio CalculatorFind the air-fuel ratio from air and fuel mass, or solve backward. Reads lambda, the equivalence ratio, and whether the mixture is rich, stoichiometric, or lean for your fuel.

Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

NOAA / NASA: U.S. Standard Atmosphere 1976
The reference model for the barometric formula and its constants.
Engineering ToolBox: Atmospheric pressure vs. altitude
Tables and the barometric relationship between altitude and pressure.