True altitude
Your true height above sea level from the altimeter reading when the air is colder or warmer than standard: high to low, look out below.
Planning and education aid. Not for primary navigation. Full disclaimer
Experimental: not yet fully verified. How results are checked
You are at 7,429 ft, 571 ft below what the altimeter shows.
- True minus indicated
- By the 4% rule
Provenance
- Computed by
- aviation.altimetry.true-altitude 1.0.0, core 0.1.0
- Model
- True − indicated = (ΔT ÷ L0) · ln(1 + L0·H ÷ (T0 + L0·H_station)) with L0 = −0.0019812 K/ft, T0 = 288.15 K, ΔT the ISA deviation, and H the indicated height above the altimeter-setting station: the ICAO Doc 8168 temperature relation, applied both colder and warmer than ISA. The 4% rule is shown beside it
- Accuracy
- Assumes the deviation holds from the station to the aircraft, as the ICAO relation does; real temperature profiles vary. For procedure altitudes, use the cold-temperature correction tool
- Notes
- 1 shown with the answer
- Cites
- International Civil Aviation Organization, Procedures for Air Navigation Services, Aircraft Operations (PANS-OPS), Doc 8168; Federal Aviation Administration, Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25C
Something look off?
How we got thisFormula, worked example, sources, and proof
Model: True − indicated = (ΔT ÷ L0) · ln(1 + L0·H ÷ (T0 + L0·H_station)) with L0 = −0.0019812 K/ft, T0 = 288.15 K, ΔT the ISA deviation, and H the indicated height above the altimeter-setting station: the ICAO Doc 8168 temperature relation, applied both colder and warmer than ISA. The 4% rule is shown beside it
Show your work
Temperature error
(ΔT ÷ L0) · ln(1 + L0·H ÷ (T0 + L0·H_station))(-20 ÷ −0.0019812) · ln(1 − 0.0019812 × 8,000 ÷ (288.15 − 0.0019812 × 0))= -571 ftTrue altitude
8,000 + -571= 7,429 ft
The same steps an agent gets from the MCP server with explain: true.
Accuracy: Assumes the deviation holds from the station to the aircraft, as the ICAO relation does; real temperature profiles vary. For procedure altitudes, use the cold-temperature correction tool
Worked example: 8,000 ft indicated in air 20 °C colder than standard. Source: add-aviation-suite true-altitude scenario: in colder air the true altitude is below the indicated one, and the difference is reported. It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.
You enter
- Indicated altitude
- 8000 ft
- ISA deviation
- -20 °C
You get
- True altitude
- 7,429 ft
- True minus indicated
- -571 ft
- By the 4% rule
- -640 ft
Review: Not yet independently reviewed by a flight instructor (CFI).
Last verified: 2026-09-19, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.
Status: version 1.0.0, core 0.1.0. See this tool in the verification report.
Checked against: 7 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.
Sources
- Procedures for Air Navigation Services, Aircraft Operations (PANS-OPS), Doc 8168, International Civil Aviation Organization, Volume II, 7th edition. Temperature correction of procedure altitudes below ISA (the 2020 equation, as cited in Transport Canada AC 500-020 section 4.8). Paid only.
- Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25C, Federal Aviation Administration, FAA-H-8083-25C. Aircraft Performance and Navigation chapters (pressure and density altitude, wind triangle).
Terms
- ISA: International Standard Atmosphere
- A reference atmosphere: 15 °C and 1013.25 hPa at sea level, cooling 6.5 °C per kilometer up to 11 km. Altimeters and performance charts are built on it. Source: Manual of the ICAO Standard Atmosphere (Doc 7488)
Experimental means this tool has not yet met the stable bar: at least 20 golden vectors, differential tests, and an independent worked example.