The E6B identity
The usual training formula treats the thickness of the pressure layer as proportional to absolute temperature:
TA = PA × (Tactual / TISA)
with temperatures in kelvin (or Rankine). ISA temperature in the troposphere is 15 °C − 1.98 °C per 1,000 ft of pressure altitude (the ICAO lapse of 0.0065 K/m). A classic example: pressure altitude 10,000 ft, OAT −25 °C (about ISA −20 °C). True altitude is about 9,250 ft — more than 700 ft below the pressure altitude. That is the “look out below” case when you are approaching terrain or a minimum altitude in cold air.
The 4-foot rule of thumb
Memory item: 4 ft of correction per 1 °C of ISA deviation per 1,000 ft of pressure altitude. A 20 °C-cold column at 10,000 ft is about 800 ft of correction, close to the exact identity. This page shows both so you can see the difference.
Sources
FAA Pilot’s Handbook of Aeronautical Knowledge (altimetry, true altitude, temperature error); FAA Instrument Flying Handbook; ICAO Doc 7488 (ISA); AIM cold-temperature altimetry. For pressure altitude itself, use the pressure altitude calculator.
Frequently asked questions
Is true altitude what my GPS altitude reads?
They are related but not the same thing. GPS height is typically a geometric / geodetic height relative to a reference ellipsoid (and may be converted to MSL with a geoid model). True altitude here is a temperature-corrected pressure height. Treat them as independent cross-checks, not as identical.
Do I correct when it is hotter than ISA?
The identity works both ways: a hot column is thicker, so true altitude is above pressure altitude. The dangerous case for terrain is almost always colder than standard.
Should I use indicated altitude or pressure altitude?
The training formula is written in terms of pressure altitude and the ISA temperature at that pressure altitude. If your altimeter is set to the current QNH, indicated altitude is already a local correction for surface pressure; cold-temperature procedures may still require a further correction, as published.