Cabin altitude from pressure calculator
At 35,000 feet with 8 psi of cabin differential, the cabin sits between 6,500 and 7,000 feet, inside the usual 8,000-foot planning limit. Both steps use the standard atmosphere, so dispatch and crews get the same answer. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Outside altitude
- 35,000
- Differential
- 8
- Result
- 6,726.5886134875
- Altitude unit
- Ft
- Outside altitude ft
- 35,000
- Outside altitude m
- 10,668
What cabin altitude will the pressurization schedule actually hold once the aircraft is at cruise pressure altitude with a given cabin differential? This cabin altitude from pressure calculator answers that dispatch and maintenance question from two numbers every crew already has on the overhead panel and the altimeter set to 29.92: outside pressure altitude and cabin ΔP. It converts the flight-level height to International Standard Atmosphere static pressure, adds the differential, and inverts the same ISA map to return cabin pressure altitude in feet and metres.
What cabin altitude means and how ISA differential pressure works
Cabin altitude is not the height of the floor above the terrain and it is not the flight level painted on the altimeter. It is the pressure altitude that corresponds to the static pressure actually present in the cabin. A transport-category pressurization system holds the hull at a higher pressure than the outside air; the difference is cabin differential pressure, ΔP equals cabin static pressure minus ambient static pressure.
Ambient static pressure is read from the International Standard Atmosphere at the aircraft outside pressure altitude—the height the altimeter shows with 29.92 inHg set—so the identity is cabin pressure equals ISA of outside altitude plus ΔP, and cabin altitude is the inverse ISA of that cabin pressure. The calculator applies that identity with the ICAO and ISO 2533 constants: sea-level pressure 101325 pascals, sea-level temperature 288.15 kelvin, tropospheric lapse 0.0065 kelvin per metre, and the tropopause at 11 kilometres.
Above the tropopause the model is isothermal at 216.65 kelvin to 20 kilometres, then a shallow plus 0.001 kelvin per metre layer to 32 kilometres so flight levels around 410 and even 510 stay inside a published atmosphere instead of a troposphere-only power law that would lie. A typical narrow-body at 35,000 feet with about 8 psi of differential lands near 6,500 to 7,000 feet cabin altitude; a higher cruise or a weaker schedule can push the cabin through the conventional 8,000 foot planning limit.
The tool does not read a cabin altimeter, model an outflow valve, or apply a non-standard temperature; you bring the pressure altitude and the ΔP the schedule is holding.
Who checks cabin altitude before a flight
Airline dispatch and performance engineers use a cabin altitude from pressure calculator when a cruise flight level and a published maximum differential must be turned into a cabin height before the flight-planning suite is open, and again when a MEL or a degraded pack schedule changes the ΔP the hull can hold. Business-aviation crews flying in the low forties need the same two-term check because tropopause ISA and a 9-plus psi schedule can sit just under or just over eight thousand feet of cabin altitude.
Maintenance organisations running a pressurization functional test already know the indicated pressure altitude and the ΔP the card calls for; they do not need a full flight-management library to confirm the cabin altimeter should read. Training organisations and type-rating briefings need a deterministic oracle that treats cabin altitude as inverse ISA of ambient plus differential, accepts feet or metres and psi or SI pressure units, and never silently swaps indicated altitude for pressure altitude.
What you can do with it
Airliner cruise pressurization check
Turn flight-level pressure altitude and the scheduled cabin ΔP into cabin feet and see whether the result stays inside 8,000 ft.
High-cruise business-jet briefing
Check a FL410 schedule with 9-plus psi of differential against the tropopause ISA so the cabin does not silently exceed eight thousand feet.
Maintenance pressurization functional test
Convert the card’s indicated pressure altitude and target ΔP into the cabin altitude the cabin altimeter should show on the ground or in the air.
FAQ
Is cabin altitude the same as flight level?
No. Flight level is the aircraft outside pressure altitude. Cabin altitude is lower whenever the hull is pressurized. With zero differential the two heights are the same; with a typical 8 psi schedule at 35,000 ft the cabin sits near 6,500–7,000 ft.
What atmosphere model does the calculator use?
ICAO / ISO 2533 International Standard Atmosphere: troposphere to 11 km, isothermal lower stratosphere to 20 km, then a +0.001 K/m layer to 32 km. Constants match the published dry-air ISA, not a local METAR or a non-standard temperature.
What cabin altitude does 8 psi give at 35,000 feet?
Between 6,500 and 7,000 feet, which is inside the conventional 8,000-foot limit. A higher cruise level or a weaker pressurization schedule would push the cabin higher, possibly past that limit.
Why does the result flag 8,000 feet?
8,000 feet is the conventional cabin ceiling used in transport-category planning. The result says whether the cabin stays within it, so a problem with the cruise level or the differential shows up before the flight.
Which pressure units can I use for the differential?
Psi by default, or inches of mercury, kilopascals, hectopascals, millibars and pascals. The outside altitude can be entered in feet or metres, and the cabin altitude is given in both.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/travel/cabin-altitude \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"outside_altitude":35000,"differential":8}'const res = await fetch("https://api.kit.forhosting.com/travel/cabin-altitude", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"outside_altitude": 35000,
"differential": 8
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/travel/cabin-altitude",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"outside_altitude": 35000,
"differential": 8
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/travel/cabin-altitude", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"outside_altitude":35000,"differential":8}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"outside_altitude":35000,"differential":8}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/travel/cabin-altitude", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"outside_altitude": 35000,
"differential": 8
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "travel.cabin_altitude",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Missing or non-finite inputs fail with invalid_input and are not billed.
Input contract, units, and response fields
Values are rounded to twelve decimal places for stable goldens. Example: thirty-five thousand feet outside and eight psi of differential yields a cabin in the high six thousands of feet, inside the 8,000 foot flag. POST a JSON body with two required finite numbers: outside_altitude, the aircraft pressure altitude, and differential, the cabin ΔP. Optional altitude_unit is ft by default or m when the height is in metres. Optional diff_unit is psi by default, or inhg, kpa, hpa, mbar, or pa when the gauge is not in pounds per square inch. Each numeric field may arrive as a JSON number or as a numeric string; missing fields, empty strings, non-finite values, a negative differential, an outside altitude outside minus two thousand to eighty thousand feet, a differential above twenty psi, invalid units, and a cabin altitude that would fall outside minus five thousand to eighty thousand feet fail with invalid_input naming the field and are not billed. On success the body echoes the cleaned inputs and reports outside_altitude_ft, outside_altitude_m, differential_psi, differential_pa, ambient_pressure_pa, ambient_pressure_psi, cabin_pressure_pa, cabin_pressure_psi, cabin_altitude_ft, cabin_altitude_m, the isa_layer of the cabin solution, and within_8000_ft against the conventional eight-thousand-foot cabin ceiling used in transport-category planning. result is an alias of cabin_altitude_ft. A formula string restates the ISA identity, method is icao_isa_cabin_from_differential, and message summarises the cabin height and the 8,000 foot check in plain language.
Where a cabin-altitude pressurization API earns its keep
Engineering notebooks that already store flight level and psid can call a tiny parse API instead of re-deriving the tropopause branch in every language binding. Keep requests small, pin cabin_altitude_ft and within_8000_ft in tests, and you get a reproducible pressurization check for briefings, LMS plugins, and edge workers without standing up an atmosphere library of your own.
Technical questions
How do I calculate cabin altitude from differential pressure?
Send outside_altitude (pressure altitude) and differential (cabin ΔP). The API evaluates ISA static pressure at the outside height, adds ΔP, and inverts the same ISA map. cabin_altitude_ft is the cabin pressure altitude.
Why is there an 8,000 ft flag?
Transport-category cabin schedules are commonly planned so cabin altitude stays at or below 8,000 ft in normal cruise. within_8000_ft is that planning check. It is not a type-certificate finding and it does not replace the aircraft flight manual.
What does the cabin altitude calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.