Crabwise · Library
Which two inputs belong in a simple pressure-altitude calculation?
Pressure altitude needs only elevation and altimeter: 1350 ft at 30.05 inHg gives 1220 ft, and 6200 ft at 29.58 inHg gives 6540 ft.
Two inputs, one rule
The pressure-altitude step in Crabwise is PA = elevation + (29.92 − altimeter) × 1000. Elevation is in feet; the altimeter setting is a pressure. Below 29.92 inHg the correction is added, above it subtracted. The app shows an inHg and hPa selector, chosen by region by default, and converts hPa to inHg with 1 inHg = 33.8638866667 hPa before applying the rule.
Temperature is not part of this step. It enters only afterwards, in the ISA deviation and density-altitude lines.
The sensitivity follows from the rule: each 0.01 inHg changes pressure altitude by 10 ft, and each hPa by about 29.529983 ft. A setting entered with a slipped digit therefore moves the result by hundreds of feet.
Worked example
A 5000 ft elevation and 29.72 inHg setting produce 5200 ft in this model. The 30°C temperature is retained for the separate atmosphere calculation; it is not part of the 200 ft setting adjustment.
- Pressure altitude from the stated simple adjustment (ft)
- 5200
Enter the two values
- Type the field elevation, or the indicated altitude, in feet.
- Select inHg or hPa to match the reported setting and type it.
- Read pressure altitude and check whether the correction was added or subtracted.
- Add the outside-air temperature only for the density-altitude result.
High and low settings
At 1350 ft with 30.05 inHg the correction is (29.92 − 30.05) × 1000 = −130, so PA is 1220 ft. At 6200 ft with 29.58 inHg the correction is +340 and PA is 6540 ft. If the low field reports 1020 hPa instead, the app converts it to about 30.120583 inHg and returns a pressure altitude of about 1149.417267 ft.
- 1350 ft, 30.05 inHg (ft)
- 1220
- 6200 ft, 29.58 inHg (ft)
- 6540
- 1020 hPa converted (inHg)
- 30.120583
- 1350 ft, 1020 hPa (ft)
- 1149.417267
The 29.58 inHg setting at the higher field equals about 1001.693768 hPa, so a station reporting in hectopascals would give the same 6540 ft once the selector matches.
Common mistakes
- Typing an hPa setting with the selector on inHg.
- Adding the correction when the setting is above 29.92.
- Calling pressure altitude density altitude.
- Entering the indicated altitude in metres while the field expects feet.
Scope
The rule is the FAA handbook's approximation, and its result is an input for later steps, not a performance figure. The FAA Pilot's Handbook below explains it.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.