Pressure Altitude Calculator

Pressure altitude from the altimeter setting

An altimeter is a barometer with a feet scale printed on it. The Kollsman window shifts that scale: set the local altimeter setting and the needles show field elevation on the ground. Set 29.92 inHg instead and they show pressure altitude, the altitude in the standard atmosphere that matches the pressure outside.

This calculator does the same shift numerically. The altimeter setting corresponds to a standard-atmosphere altitude of its own, and the difference is added to the field elevation:

• a setting below 29.92 means lower pressure, so pressure altitude is above field elevation;
• a setting above 29.92 means higher pressure, so pressure altitude is below field elevation.

The worked example and the FAA table

With a field elevation of 5,883 feet and an altimeter setting of 30.10 inHg, the correction is −165 feet and the pressure altitude is 5,718 feet. That is PHAK Sample Problem 1 (FAA-H-8083-25C, chapter 11), and the −165 feet comes straight from the handbook's altimeter-correction table (Figure 11-3).

The calculator reproduces that whole table, from 28.00 to 31.00 inHg, within 1.5 feet on every row. Here are a few rows for reference:

Quick estimate for the knowledge test

The approximation pilots learn is 1,000 feet per inch of mercury: (29.92 − 30.10) × 1,000 = −180 feet, giving 5,703 feet. Knowledge-test questions usually accept it, and the result shows it next to the exact value. The difference is small at typical settings: 15 feet here.

Station pressure

The result also lists station pressure, the actual pressure at the field. Density altitude and true airspeed start from it, and the National Weather Service's density-altitude formula asks for it. The density altitude calculator takes the same inputs plus temperature, and the standard atmosphere calculator gives standard pressure at any altitude.

High to low, look out below

The same relationship explains the classic altimeter trap. If you fly from an area where the altimeter setting is 30.20 toward one where it is 29.90 without resetting it, the altimeter reads about 280 feet higher than your real height above sea level; the rule of thumb says 300. Flying from high pressure to low pressure, the airplane is lower than the altimeter shows. Cold air does the same: in air colder than standard the altimeter over-reads too. Update the setting from nearby stations as you go, and keep extra clearance over terrain on cold days.

Pressure altitude and flight levels

In the United States, pilots climbing through 18,000 feet MSL set 29.92 inHg, and altitudes from there up are flown as flight levels: pressure altitudes in hundreds of feet. Flight level 180 is a pressure altitude of 18,000 feet. Everyone at those levels uses the same setting, so traffic stays separated even though true altitudes drift with the weather.