Pressure Altitude Calculator
Easily calculate pressure altitude based on indicated altitude and altimeter setting.
Calculate Pressure Altitude
Pressure Altitude vs. Altimeter Setting
Standard Atmosphere Table
| Altitude (ft) | Temperature (°C) | Pressure (inHg) | Pressure (hPa) |
|---|---|---|---|
| 0 (Sea Level) | 15.0 | 29.92 | 1013.25 |
| 1,000 | 13.0 | 28.86 | 977.16 |
| 2,000 | 11.0 | 27.82 | 942.13 |
| 5,000 | 5.1 | 24.89 | 843.11 |
| 10,000 | -4.8 | 20.58 | 696.81 |
| 18,000 | -20.7 | 14.95 | 506.10 |
| 25,000 | -36.6 | 11.10 | 376.00 |
| 36,000 | -56.5 | 6.68 | 226.32 |
What is Pressure Altitude?
Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question. In simpler terms, it’s the altitude your aircraft *would* be at if the altimeter was set to the standard sea level pressure of 29.92 inches of Mercury (inHg) or 1013.25 hectopascals (hPa), under standard atmospheric conditions.
Pilots, meteorologists, and aviation enthusiasts need to understand and calculate pressure altitude because it’s crucial for aircraft performance calculations, flight planning, and understanding air density. Aircraft performance charts (takeoff distance, climb rate, landing distance) are often based on pressure altitude and density altitude (which is pressure altitude corrected for non-standard temperature).
A common misconception is that pressure altitude is the same as the aircraft’s true altitude above sea level. This is only true if the local altimeter setting happens to be exactly 29.92 inHg and the atmosphere matches ISA conditions.
Who Should Use It?
- Pilots: Essential for performance calculations, determining flight levels above the transition altitude, and understanding aircraft limitations. Knowing how to calculate pressure altitude is fundamental.
- Flight Planners and Dispatchers: To plan safe and efficient routes considering aircraft performance at various pressure altitudes.
- Meteorologists: To analyze atmospheric conditions and pressure systems.
- Aerospace Engineers: When designing and testing aircraft, performance is often referenced against pressure altitude.
Pressure Altitude Formula and Mathematical Explanation
The basic formula to calculate pressure altitude when you know the indicated altitude and the current altimeter setting (QNH) is based on the standard pressure lapse rate near sea level.
The relationship is approximately 1000 feet of altitude change for every 1 inch of Mercury change in pressure (or about 27-30 feet per hectopascal). When the local altimeter setting is lower than the standard 29.92 inHg, the pressure altitude will be higher than the indicated altitude, and vice-versa.
Formula:
Pressure Altitude (ft) = Indicated Altitude (ft) + [ (29.92 – Altimeter Setting (inHg)) * 1000 ]
Where:
- Indicated Altitude is the altitude read directly from the altimeter when set to the local barometric pressure (QNH).
- Altimeter Setting (QNH) is the current atmospheric pressure at sea level, given in inches of Mercury (inHg).
- 29.92 is the standard sea level pressure in inches of Mercury.
- 1000 is the approximate number of feet change in altitude per 1 inHg pressure change near sea level in the standard atmosphere.
If you are working with hectopascals (hPa) or millibars (mb), the standard pressure is 1013.25 hPa, and the approximate altitude change is about 27 to 30 feet per hPa. The formula becomes:
Pressure Altitude (ft) ≈ Indicated Altitude (ft) + [ (1013.25 – Altimeter Setting (hPa)) * 27 ] (using 27 ft/hPa)
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Pressure Altitude | Altitude in ISA corresponding to the sensed pressure | feet (ft) or meters (m) | -2,000 to >50,000 ft |
| Indicated Altitude | Altitude read from altimeter set to local QNH | feet (ft) or meters (m) | 0 to >50,000 ft |
| Altimeter Setting (QNH) | Local barometric pressure corrected to sea level | inHg or hPa (mb) | 28.00 – 31.00 inHg (948 – 1050 hPa) |
| Standard Pressure | ISA sea level pressure | inHg or hPa (mb) | 29.92 inHg or 1013.25 hPa |
| Pressure Difference Factor | Approx. altitude change per unit pressure change | ft/inHg or ft/hPa | ~1000 ft/inHg or ~27-30 ft/hPa |
Practical Examples (Real-World Use Cases)
Example 1: Airport at Moderate Elevation
An aircraft is at an airport with a field elevation (indicated altitude on the ground) of 3,500 feet. The local altimeter setting (QNH) reported by ATIS is 29.70 inHg.
- Indicated Altitude = 3,500 ft
- Altimeter Setting = 29.70 inHg
Pressure Difference = 29.92 – 29.70 = 0.22 inHg
Altitude Correction = 0.22 * 1000 = 220 feet
Pressure Altitude = 3,500 + 220 = 3,720 feet
The pilot would use 3,720 feet (and the current temperature) to calculate density altitude and determine aircraft performance for takeoff.
Example 2: High Altimeter Setting
An aircraft is flying at an indicated altitude of 10,000 feet. The nearest weather station reports an altimeter setting of 30.42 inHg.
- Indicated Altitude = 10,000 ft
- Altimeter Setting = 30.42 inHg
Pressure Difference = 29.92 – 30.42 = -0.50 inHg
Altitude Correction = -0.50 * 1000 = -500 feet
Pressure Altitude = 10,000 + (-500) = 9,500 feet
In this case, because the local pressure is higher than standard, the pressure altitude is lower than the indicated altitude.
How to Use This Pressure Altitude Calculator
Using our calculator to calculate pressure altitude is straightforward:
- Enter Indicated Altitude: Input the altitude your altimeter is currently showing (or the field elevation if on the ground) in feet.
- Enter Altimeter Setting: Input the current altimeter setting (QNH) provided by the local weather report or ATIS, in inches of Mercury (inHg).
- View Results: The calculator will instantly display the calculated Pressure Altitude, the Pressure Difference from standard, and the Altitude Correction applied.
- Reset: Use the “Reset” button to return to the default values.
- Copy: Use the “Copy Results” button to copy the inputs and results to your clipboard.
The results help you understand the altitude your aircraft “feels” in terms of standard atmospheric pressure, which is a key step before calculating density altitude for performance figures.
Key Factors That Affect Pressure Altitude Results
Several factors influence how we calculate pressure altitude and its significance:
- Indicated Altitude: This is your starting point. The higher your indicated altitude, the higher your pressure altitude will generally be, given the same altimeter setting.
- Altimeter Setting (QNH): This is the most direct variable other than indicated altitude. A lower QNH than standard (29.92 inHg) means the local pressure is lower, so the pressure altitude will be higher than indicated. A higher QNH means higher local pressure, so pressure altitude will be lower than indicated.
- Standard Pressure Reference: The calculation uses 29.92 inHg (or 1013.25 hPa) as the baseline. Any deviation from this in the local QNH directly affects the result.
- Accuracy of Altimeter Setting: An incorrect or outdated altimeter setting will lead to an incorrect pressure altitude calculation. Always use the most current local setting.
- Pressure Lapse Rate Approximation: The 1000 ft/inHg (or 27-30 ft/hPa) is an approximation based on the standard atmosphere near sea level. The actual lapse rate changes with altitude and temperature, but for practical purposes below the tropopause, this is a standard and accepted approximation for pressure altitude calculation before applying temperature correction for density altitude.
- Instrument Error: The aircraft’s altimeter itself may have some instrument error, affecting the indicated altitude reading.
Frequently Asked Questions (FAQ)
A: Pressure altitude is altitude corrected for non-standard pressure. Density altitude is pressure altitude corrected for non-standard temperature. Density altitude is what directly affects aircraft and engine performance. You need to calculate pressure altitude first to find density altitude.
A: It’s the basis for aircraft performance calculations, is used above the transition altitude (as flight levels), and is needed to calculate density altitude.
A: The altitude at or above which pilots set their altimeters to 29.92 inHg (or 1013 hPa), and fly “flight levels” (pressure altitude / 100). In the US, it’s 18,000 feet MSL.
A: From Airport Terminal Information Service (ATIS), METAR weather reports, or Air Traffic Control (ATC).
A: Yes, if you are below sea level and the altimeter setting is very high, or even above sea level with an extremely high altimeter setting, the calculated pressure altitude could be negative.
A: No, the basic formula for pressure altitude corrects for non-standard pressure only. Temperature is used *after* you calculate pressure altitude to find density altitude.
A: Convert it to inHg (1 hPa ≈ 0.02953 inHg) before using this calculator, or use the hPa-based formula: PA (ft) ≈ IA (ft) + [(1013.25 – QNH(hPa)) * 27]. Our calculator uses inHg.
A: It’s a good approximation for lower altitudes in the standard atmosphere. The actual rate varies slightly with altitude and temperature profile.
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