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How to Calculate an Altimeter Setting

An altimeter is an aneroid barometer that translates change in pressure into altitude. Changes in barometric pressure associated with changes in weather cause changes in altimeter readings. Altimeters need to be adjusted to compensate for these pressure changes. Altimeters use barometric pressure readings adjusted to sea level. Entering the sea level barometric pressure produces the current altitude readout on the instrument. In general, media reports of barometric pressure have already been corrected to sea level.

Things You'll Need

Local barometer setting
Altimeter

Instructions

- 1
  Obtain the current barometric pressure and determine if a correction is required. Barometric pressure obtained from a flight services station or a weather broadcast has already been corrected to sea level. If the barometric pressure has been corrected to sea level, enter the sea level pressure into the kollsman window of the altimeter and the altimeter will read the present elevation. Entering the correct elevation into the altimeter will cause the altimeter to indicate the sea level barometric pressure in the kollsman window.
- 2
  Adjust local barometric pressure for elevation. Add .1 inches of mercury (in/Hg) to your local reading for every 100 feet above sea level. For example, if you are at 1,500 above sea level, add 1.5 in/Hg to the local barometric pressure to adjust to sea level pressure. If your local pressure at 1,500 feet is 28.00 in/Hg add 1.5 to 28.00 to obtain a sea level pressure of 29.50 in/Hg.
- 3
  Enter the adjusted pressure into the barometer by setting the pressure viewed through the kollsman window to the sea level pressure you calculated.
- 4
  Verify the altimeter setting. The altimeter should indicate the correct elevation once the sea level pressure has been entered.

Aviation