Altimeter Error Calculator (Wrong Setting "High to Low", Cold-Temperature Correction, True Altitude)

How it works

setting error: true = indicated + (actual setting − set setting) × 1,000 ft/inHg · cold correction (ICAO simplified): ΔH = H × (ISA_station − T) ÷ (273.15 + T − 0.5 × 0.0019812 × H), H = height above the station, ISA_station = 15 − 0.0019812 × elevation · true ≈ pressure-corrected − ΔH

An altimeter is a barometer that assumes a standard atmosphere. Set it to a pressure higher than the real one and it reads high; fly into colder-than-standard air and the pressure levels are packed closer together, so it reads high again — both errors put you lower than you think, which is why pilots learn "high to low, look out below". The setting error is about 1,000 feet per inch of mercury; the temperature error grows with height above the station and with how cold it is, and ICAO publishes the correction that should be added to minimum altitudes on cold days.

Worked example

Flying at an indicated 3,000 ft with 30.12 still set when the local setting is 29.72 puts you 400 ft lower — 2,600 ft. At −20 °C over a 1,200 ft station (ISA there is 12.6 °C, so 32.6 °C colder) the 1,400 ft above the station over-read by about 180 ft, so the true altitude is roughly 2,420 ft — about 80 ft BELOW the top of 2,500 ft terrain the altimeter says you clear by 500 ft.

Frequently asked questions