Air Pressure at Altitude Calculator

About the Air Pressure at Altitude Calculator

Atmospheric pressure decreases with altitude because there is less air above pressing down. The International Standard Atmosphere (ISA) defines how pressure, temperature, and density vary with height: in the troposphere (0–11 km), temperature drops at 6.5°C per km and pressure follows a power-law profile; in the lower stratosphere (11–20 km), temperature is constant and pressure drops exponentially.

This calculator implements the ISA barometric formula for altitudes from sea level to 47 km. You can adjust the sea-level pressure for current weather conditions, apply temperature deviations from the standard profile, and include relative humidity for a moist-air density correction. The outputs include pressure in six unit systems, temperature, air density, and the altitude-corrected boiling point of water.

Pilots use altimeter settings (inHg or hPa) to convert pressure to altitude. Engineers need air density for drag and lift calculations. Hikers and climbers want to know the boiling point of water at camp altitude. This calculator serves all those needs in a single interface.

When This Page Helps

Whether you are a pilot checking density altitude, an engineer computing drag at altitude, or a mountaineer planning a camp stove, this calculator gives accurate atmospheric properties at any height using the internationally accepted ISA model.

How to Use the Inputs

1. Enter the altitude in meters, feet, or kilometers.
2. Use a preset for well-known locations (Denver, Everest, jet cruise altitude).
3. Optionally adjust the ISA temperature offset for non-standard days.
4. Enter the actual sea-level pressure to match local weather conditions.
5. Enter relative humidity for a more accurate density estimate.
6. Read pressure in kPa, atm, psi, inHg, and mmHg, along with temperature and density.
7. Consult the altitude profile table for a full overview of atmospheric conditions.

Example Calculation

Tips & Best Practices

• Density altitude — the altitude at which ISA density equals actual density — is crucial for aircraft take-off performance.
• Use the inHg output for US altimeter settings; hPa (= mbar = kPa × 10) for ICAO settings.
• At Everest summit (~8 849 m), pressure is about 33% of sea-level — roughly one-third the oxygen.
• For cooking at altitude, increase cooking times or use a pressure cooker to raise the boiling point.
• Commercial jets cruise at 10–12 km where pressure is only 25–30% of sea level; cabins are pressurized to ~6 000 ft equivalent.

When To Use This Calculator

Calculate atmospheric pressure, temperature, and air density at any altitude using the International Standard Atmosphere (ISA) model. Supports humidity correction. Use it when you need a repeatable calculation in the physics / fluid category and want the setup, result, and supporting values kept together. This is especially helpful when small input changes, unit choices, or rounding decisions can change the final number.

How To Check The Result

Start by confirming that the inputs match the formula shown on the page. Then compare the main output with the worked example and any secondary values shown by the calculator. If the result will be used in another calculation, keep extra precision until the final step and record the assumptions beside the number.

Practical Notes

Treat the result as a calculation aid rather than a substitute for context. For schoolwork, include the formula and substitution steps. For planning, technical, financial, or health-related decisions, verify important numbers against primary records, current rules, or a qualified professional before acting on them.

Frequently Asked Questions

• How much does pressure drop per 1 000 m?

  Near sea level, pressure drops roughly 12 kPa per 1 000 m (about 12%). The rate decreases with altitude because the atmosphere thins exponentially, not linearly.

• Why does water boil at a lower temperature at altitude?

  Boiling occurs when vapor pressure equals atmospheric pressure. Lower atmospheric pressure means less vapor pressure is needed, so water boils at a lower temperature — roughly 3.4°C less per 1 000 m.

• What is ISA deviation?

  ISA deviation (ISA±°C) is the difference between actual temperature and the standard atmosphere temperature at that altitude. Pilots report ISA+10 or ISA−5 to describe non-standard conditions.

• Why does humidity affect air density?

  Water vapor (M = 0.018 kg/mol) is lighter than dry air (M = 0.029 kg/mol). Humid air is therefore slightly less dense than dry air at the same pressure and temperature, affecting aircraft performance.

• How do altimeters convert pressure to altitude?

  An altimeter is a calibrated barometer. It measures ambient pressure and converts it to an altitude using the ISA profile. Pilots set the local sea-level pressure (QNH) to get accurate altitude readings.

• Is the ISA model accurate?

  ISA is an idealized average. Real atmospheric conditions vary with weather, latitude, and season. For precise work, use radiosondes or numerical weather prediction data.