Aircraft Cabin Altitude & Pressurization Calculator
Enter pressure altitude, cabin altitude, and differential pressure values to solve the pressurization question you have. The calculator uses the ISA pressure-altitude relationship so the answer is reversible across the three modes.
Cabin pressure altitude5,373
- Aircraft pressure altitude (ft)
- 35,000
- Cabin pressure altitude (ft)
- 5,373
- Required or applied differential (psi)
- 8.6
- Outside ambient pressure
- 3.46 psi / 23.8 kPa
- Cabin absolute pressure
- 12.06 psi / 83.1 kPa
- Differential-pressure margin (psi)
- 0
- Differential utilization
- 100%
- Pressurization result
- Applied differential is within the entered maximum limit.
Uses the ISA pressure-altitude model through 65,000 ft and treats pressure differential as static cabin pressure minus outside pressure.
How to use this calculator
- Choose whether to calculate cabin altitude, required differential, or maximum aircraft pressure altitude.
- Enter aircraft pressure altitude when the selected mode uses the aircraft altitude.
- Enter the cabin pressure altitude target when solving for required differential or maximum altitude.
- Enter the applied or maximum allowable differential pressure in psi.
- Read the pressure, margin, utilization, and status lines before using the result.
How the pressurization calculation works
Cabin altitude is another way to state cabin absolute pressure. A lower cabin altitude means higher cabin pressure; a higher aircraft pressure altitude means lower outside ambient pressure. Differential pressure is the cabin absolute pressure minus the outside ambient pressure.
This calculator converts pressure altitude to pressure with the ISA model. For altitudes at or below 11,000 m, it uses:
P(h) = P0 x (1 - Lh / T0)^(g0M / RL)
For 11,000 m to 20,000 m, it uses:
P(h) = P11 x exp[-g0M(h - 11000) / (R x T11)]
Here P0 is 101325 Pa, T0 is 288.15 K, L is 0.0065 K/m, g0 is 9.80665 m/s^2, M is 0.0289644 kg/mol, and R is 8.3144598 J/(mol K). The calculator also inverts those equations to turn a calculated pressure back into an ISA pressure altitude.
What each mode solves
Cabin-altitude mode first finds outside pressure at the aircraft pressure altitude. It adds the applied differential pressure, caps the cabin pressure at standard sea-level pressure, and then converts that cabin pressure back to cabin altitude. Required-differential mode finds outside pressure at the aircraft altitude and cabin pressure at the target cabin altitude, then subtracts outside pressure from cabin pressure. Maximum-altitude mode subtracts the maximum allowable differential from the target cabin pressure, then finds the aircraft pressure altitude where ambient pressure reaches that value.
The most important inputs are differential pressure limit and the target cabin altitude. A small change in allowable differential can move the maximum pressure altitude by thousands of feet near high-altitude cruise. A target cabin altitude above the aircraft altitude produces a negative differential; that is an underpressure condition, not normal pressurization.
What this leaves out
This is a static ISA pressure calculation. It does not model a real controller schedule, cabin climb rate, descent schedule, landing-field elevation, relief-valve tolerance, leaks, outflow-valve behavior, temperature effects, or nonstandard altimeter setting. Use pressure altitude for the aircraft altitude input; GPS altitude and geometric altitude are different quantities.
Worked example
Suppose the aircraft is at 35,000 ft pressure altitude and the applied differential is 8.6 psi. Standard outside pressure at 35,000 ft is about 3.46 psi. Cabin absolute pressure is therefore 3.46 + 8.60 = 12.06 psi, or about 83.1 kPa.
Inverting the ISA pressure equation gives a cabin pressure altitude of about 5,373 ft. With an 8.6 psi maximum differential entered, utilization is 100% and the margin is 0 psi.
If instead you target an 8,000 ft cabin at 35,000 ft, the cabin pressure is about 10.92 psi and the required differential is about 10.92 - 3.46 = 7.46 psi.
Common questions
What is cabin altitude?
Cabin altitude is the ISA altitude that corresponds to the pressure inside the cabin. A cabin altitude of 8,000 ft means the cabin pressure is approximately the same as standard atmospheric pressure at 8,000 ft, even though the aircraft may be much higher.
How is cabin altitude calculated from differential pressure?
The calculator finds outside pressure from aircraft pressure altitude, adds the applied cabin-to-ambient differential pressure, and converts that cabin pressure back to an ISA altitude. If the sum would exceed standard sea-level pressure, the displayed cabin altitude is capped at sea level for this model.
What is the difference between cabin altitude and aircraft altitude?
Aircraft pressure altitude describes the outside static pressure around the airplane. Cabin altitude describes the pressure maintained inside the cabin. Pressurization keeps cabin altitude lower than aircraft altitude by holding cabin pressure above outside pressure.
How much differential pressure is required at FL350?
Under ISA conditions, 35,000 ft has an outside pressure of about 3.46 psi. An 8,000 ft cabin has a pressure of about 10.92 psi, so the required differential is about 7.46 psi.
Should I use pressure altitude or indicated altitude?
Use pressure altitude for the aircraft input because the ISA equations are pressure-altitude equations. Indicated altitude depends on the altimeter setting, and GPS altitude is geometric height rather than static pressure.
Where can I find the maximum allowable differential pressure?
Use approved aircraft data such as the POH, AFM, type certificate data sheet, or maintenance manual. Do not infer the limit from a typical aircraft value or from this calculator.