Altimeter setting is the barometric pressure value a pilot dials into an altimeter so the instrument displays the aircraft's true altitude above mean sea level (MSL).
How It Works#
An altimeter is a pressure-sensitive instrument. It measures the air pressure outside the aircraft and converts that reading into an altitude display. The problem is that sea-level pressure is not constant. It changes with weather systems moving across the region.
The altimeter setting corrects for this variation. A pilot sets the current local pressure (in inches of mercury in the US, or hectopascals elsewhere) into a small window on the instrument face called the Kollsman window. The altimeter then adjusts its altitude reading to match real-world conditions.
In the US, altimeter settings are reported in inches of mercury (inHg). Standard sea-level pressure is 29.92 inHg, or 1013.25 hPa in the metric system used internationally. When flying above 18,000 feet MSL in the US, all pilots set 29.92 inHg, called standard pressure. Below 18,000 feet, pilots use the local setting provided by air traffic control (ATC) or an automated weather station.
Example in Aviation#
A pilot departs an airport where ATC reports an altimeter setting of 29.85 inHg. She dials 29.85 into the Kollsman window. Her altimeter now reads the airport elevation correctly on the ground, confirming the setting is accurate before takeoff.
As she flies toward her destination 150 miles away, ATC gives her a new setting of 30.02 inHg. She updates the Kollsman window. Without that update, her altimeter would read slightly lower than her actual altitude, creating a false sense of terrain clearance.
Why It Matters#
An incorrect altimeter setting is a safety hazard. If the displayed altitude is off by even a few hundred feet, a pilot flying an instrument approach or navigating near terrain may not have the clearance she thinks she has. Controlled flight into terrain (CFIT) is one of aviation's deadliest accident categories, and altimeter errors contribute to that risk.
For student pilots, understanding altimeter setting builds the foundation for instrument flying. It also explains why ATC routinely passes along updated pressure readings during flight. Every handoff to a new ATC sector is a chance to update the setting and stay accurate.
Key Takeaways#
- Altimeter setting corrects for local barometric pressure so your altitude display is accurate.
- In the US, settings are in inHg; internationally, hPa (hectopascals) is standard.
- Below 18,000 feet MSL in the US, use the current local altimeter setting.
- Above 18,000 feet MSL in the US, all pilots set standard pressure: 29.92 inHg.
- An outdated setting can put you hundreds of feet lower than your altimeter shows.