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Pressure Altitude Calculator

Converts between pressure and altitude in the ICAO International Standard Atmosphere: pressure altitude from station pressure (or from QNH/altimeter setting plus field elevation), and the ISA pressure and temperature at a given altitude.

When to use

You need pressure altitude for aviation performance, the standard pressure or temperature at an altitude, or station pressure from a sea-level-reduced reading and elevation.

Do not use when: You need true altitude on a non-standard day (that requires temperature correction), altitudes above 11 km (stratosphere), or the density altitude (use air-density).

Formula

p(h) = 1013.25 × (1 − 0.0065·h/288.15)^5.25578 hPa; h(p) = 44330.8 × [1 − (p/1013.25)^0.190267] m; station pressure from QNH: p_station = QNH × (1 − 0.0065·elevation/288.15)^5.25578; T_ISA = 15 − 0.0065·h °C

ICAO Standard Atmosphere troposphere (sea level 1013.25 hPa, 15 °C, lapse 6.5 K/km, g = 9.80665 m/s², M = 28.9644 g/mol), valid to 11 000 m (226.32 hPa). The NWS pressure-altitude formula uses slightly different rounded constants (145366.45 ft, exponent 0.190284) and agrees within 0.1 %.

Inputs

Parameter	Type	Unit	Required	Description
`pressure`	number		no	Station pressure in pressure_unit, or the QNH/altimeter setting when elevation_m is also given. Omit to only evaluate altitude_m. Range: > 0, ≤ 1200
`pressure_unit`	enum: hpa \| kpa \| inhg \| mmhg		default hpa	Unit of the pressure input.
`elevation_m`	number	m	no	Elevation of the station. When given, `pressure` is interpreted as QNH (sea-level-reduced altimeter setting) and station pressure is derived. Range: ≥ -500, ≤ 9000
`altitude_m`	number	m	no	Optional geopotential altitude at which to report the ISA pressure and temperature. Range: ≥ -500, ≤ 11000

Outputs

Output	Type	Unit	Description
`pressure_altitude_m`	number	m	ISA altitude at which the standard pressure equals the station pressure.
`pressure_altitude_ft`	number	ft	Pressure altitude in feet.
`station_pressure_hpa`	number	hPa	Absolute pressure at the station (input, or derived from QNH and elevation).
`station_pressure_inhg`	number	inHg	Station pressure in inches of mercury.
`isa_pressure_hpa`	number	hPa	Standard-atmosphere pressure at altitude_m (only when altitude_m is given).
`isa_pressure_inhg`	number	inHg	Same in inches of mercury.
`isa_temperature_c`	number	°C	15 °C minus 6.5 °C per 1000 m.

Example

Station pressure 900 hPa: {"pressure":900,"pressure_unit":"hpa"} → {"pressure_altitude_m":988.5,"pressure_altitude_ft":3243,"station_pressure_hpa":900,"station_pressure_inhg":26.577}

ISA at 1500 m: {"altitude_m":1500} → {"isa_pressure_hpa":845.56,"isa_pressure_inhg":24.97,"isa_temperature_c":5.25}

GET https://tttkmbb.com/api/v1/calculate/pressure-altitude?pressure=900&pressure_unit=hpa

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/pressure-altitude (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/pressure-altitude · Markdown: https://tttkmbb.com/weather/pressure-altitude.md · JSON definition: https://tttkmbb.com/weather/pressure-altitude.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="pressure-altitude"
OpenAPI operationId: convert_pressure_altitude
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – International Standard Atmosphere (ICAO Doc 7488) (reference)
NWS El Paso – Pressure altitude formula (government)

FAQ

What is the difference between QNH and station pressure?

Station pressure is the actual pressure at the site; QNH (altimeter setting) is that pressure reduced to sea level through the ISA so that an altimeter reads field elevation. Give elevation_m to convert between them.

Is pressure altitude the true altitude?

Only on a standard day. Warmer-than-ISA air makes true altitude higher than pressure altitude (about 4 ft per 1000 ft per °C of deviation); aviation performance charts nevertheless use pressure altitude.

Why is 1013.25 hPa the reference?

It is the ISA sea-level pressure (29.92 inHg); set the altimeter to it and it displays pressure altitude directly.

Related calculators

Air Density Calculator — Density altitude from temperature, pressure and humidity.
Boiling Point at Altitude Calculator — Boiling point of water at the resulting pressure.
Pressure Converter — Convert pressure units (hPa, inHg, psi, kPa).