text: Aircraft operating at higher altitudes ( at or above what is called the transition altitude , which varies by country ) set their barometric altimeters to a standard pressure of 29.92 inHg ( 1 atm = 29.92 inHg ) or 1,013.25 hPa ( 1 hPa = 1 mbar ) regardless of the actual sea level pressure , with inches of mercury used in the U.S. and Canada .
simplified: Airplanes that are flying at high altitudes , or that are flying higher than what is called the Transition Altitude , which varies by country set their barometric altimeters , which control the air pressure in their plane , to a standard pressure of 29.92 inHg or 1,013.2 hPa ( 1 hPa = 1 mbar ) no matter what the actual sea level pressure is , in inches of mercury used in the U.S. and Canada , and in other units in other countries .