The instrument panel
Pressure & True Altitude Converter
Altimeter-setting scenarios: pressure altitude from the exact NWS formula, ISA deviation at your field, and the cold-day true-altitude estimate.
Pressure altitude
PRESSURE ALTITUDE
- CORRECTION
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- ISA TEMP HERE
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- ISA DEVIATION
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True altitude on a nonstandard day
Rule-of-thumb estimate: about 4 ft of altimeter error per 1,000 ft above the reporting station for each °C away from ISA.
ESTIMATED TRUE ALTITUDE
For training and reference · verify with official sources and your POH before flight · no tool on this site renders a go / no-go judgment
Frequently asked questions
What is pressure altitude?
Pressure altitude is what your altimeter reads with 29.92 inHg set — your height above the standard datum plane. It is the entry point for performance charts, the altitude reference at and above FL180, and the first step of every density altitude problem.
How much does one inch of altimeter setting change pressure altitude?
Roughly 1,000 ft per inch of mercury is the training rule. The real standard atmosphere is slightly sub-linear — the published FAA conversion table runs from roughly 910 to 965 ft per inch across the 28.0–31.0 range, steepening toward the low end, which is what the exact formula here reproduces.
Why does true altitude differ from indicated altitude?
The altimeter is calibrated for the standard atmosphere. On a colder-than-standard day the pressure levels squeeze closer together and the altimeter reads higher than you are — the old warning “from high to low, look out below.” The estimate here uses the training rule of about 4 ft of error per 1,000 ft above the station for each °C of ISA deviation.
Is this the same as density altitude?
No — pressure altitude ignores temperature. Density altitude is pressure altitude corrected for nonstandard temperature (and optionally humidity). For performance questions, follow up with the density altitude calculator.