How do I get pressure altitude?

PHAK Chapter 11 gives three methods: set the altimeter to 29.92 inHg and read it; apply the correction for the reported altimeter setting; or use a flight computer. Pressure altitude is the height above the standard datum plane, 29.92 inHg.

What is standard temperature, and what does “20 degrees above standard” mean?

PHAK Chapter 11: standard sea-level temperature is 15 °C (59 °F) and standard pressure is 29.92 inHg. Temperature decreases at about 2 °C (3.5 °F) per thousand feet up to 36,000 feet. At 5,000 feet, standard temperature is about 5 °C. “20 °C above standard” on that day is about 25 °C, not an outside air temperature of 20 °C.

Is there one formula in the PHAK, such as 120 feet per degree?

Chapter 11 of FAA-H-8083-25C does not give a feet-per-degree formula. It says to use temperature correction from tables or graphs, or a flight computer, and it works an example: pressure altitude 5,000 feet and a temperature 20 °C above standard produces a density altitude above 7,000 feet. Use the chart or the computer on the oral. Do not substitute a rule of thumb the handbook does not state.

What does high density altitude do to the airplane?

PHAK Chapter 11: as air becomes less dense it reduces power, because the engine takes in less air; thrust, because the propeller is less efficient in thin air; and lift, because the thin air exerts less force on the airfoils. The airplane performs as though it were operating at the density altitude. The handbook’s example takes a 790-foot ground roll under standard temperature at that 5,000-foot pressure altitude and says the ground run may be closer to 1,000 feet on the 20 °C-hot day.

Where does humidity fit, and what does 91.103 add?

PHAK Chapter 11 lists high elevation, low pressure, high temperature, and high humidity as conditions that result in a high density altitude. The sample chart uses pressure altitude and outside air temperature. The handbook says humidity is usually not essential to the calculation, but moist air is less dense, there is no rule of thumb or chart for that effect, and it still has to be considered. Section 91.103 requires the PIC, before any flight, to know runway lengths at airports of intended use and the takeoff and landing distance data in the approved flight manual, or other reliable performance information for aircraft that are not required to have one, using expected airport elevation, runway slope, gross weight, wind, and temperature.

What is density altitude?

Density altitude is pressure altitude corrected for nonstandard temperature. The Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25C, Chapter 11, defines it as the altitude in the standard atmosphere corresponding to a particular value of air density. The airplane performs as though it were at that altitude. Find pressure altitude, then correct it for temperature with a flight computer or the density-altitude chart. High density altitude means thinner air, so the engine makes less power, the propeller produces less thrust, and the wings produce less lift. 14 CFR 91.103 requires takeoff and landing distance information before the flight.

Study aid, not legal advice. Regulations, the AIM, and handbook editions change. Check the current text and confirm anything that affects a flight or a checkride with your CFI.

The definition

FAA-H-8083-25C, Chapter 11 (the current Pilot’s Handbook of Aeronautical Knowledge; the FAA handbook page also lists an October 2025 addendum) says density altitude is “the altitude in the standard atmosphere corresponding to a particular value of air density,” and that it “is pressure altitude corrected for nonstandard temperature.”

Under standard conditions, pressure altitude and density altitude are the same level. When temperature or pressure is nonstandard, the altimeter does not show the density altitude the performance charts assume. The handbook’s line to remember: regardless of the actual altitude, the aircraft performs as though it were at the existing density altitude. As density increases, density altitude is lower and performance increases. As density decreases, density altitude is higher and performance decreases.

The standard atmosphere in that chapter is 15 °C (59 °F) and 29.92 inHg (1013.2 mb) at sea level. The standard temperature lapse rate is approximately 3.5 °F or 2 °C per thousand feet up to 36,000 feet. A standard pressure lapse rate is approximately 1 inHg per 1,000 feet of altitude gain to 10,000 feet.

How to compute it

The handbook’s order is: find pressure altitude, then correct that altitude for nonstandard temperature.

Set 29.92 inHg and read pressure altitude, or correct the field elevation for the reported altimeter setting, or use a flight computer.
Enter that pressure altitude and the outside air temperature on an E6B-style flight computer, or on the density-altitude chart (the handbook’s Figure 11-4).

Worked example from the chapter, not a formula to invert: with the altimeter set to 29.92, pressure altitude is 5,000 feet. At standard temperature the AFM/POH ground run may be 790 feet. If the temperature is 20 °C above standard, the density level is above 7,000 feet and the ground run may be closer to 1,000 feet. “20 °C above standard” is not the same statement as “the OAT is 20 °C.”

The chapter’s chart sample (Figure 11-22): airport elevation 5,883 feet, OAT 70 °F, altimeter 30.10 inHg. The pressure-altitude conversion for 30.10 is −165, so pressure altitude is 5,718 feet. Reading the chart, the approximate density altitude is 7,700 feet. That 7,700 is the handbook’s chart reading for that sample, which is why the oral expects the chart or the computer rather than mental arithmetic.

High density altitude comes from high elevation, low atmospheric pressure, high temperature, high humidity, or a combination. The chart corrects pressure altitude and temperature. The handbook says there is no rule of thumb or chart for humidity, and that high humidity still decreases performance.

What you have to do with it before takeoff

Section 91.103 says each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. For any flight, that information must include runway lengths at airports of intended use and takeoff and landing distance data: the data in the approved Airplane or Rotorcraft Flight Manual when one is required, or other reliable information appropriate to the aircraft, relating to performance under expected airport elevation and runway slope, gross weight, and wind and temperature.

The temperature and elevation you put into the POH chart are how you comply with that paragraph on a hot day. A chart entered with sea-level standard temperature, at a high airport, is not the information 91.103 is talking about. Section 61.105(b)(8) lists “effects of density altitude on takeoff and climb performance” as a private pilot knowledge area.

For a flight under IFR, or a flight not in the vicinity of an airport, 91.103(a) also requires weather reports and forecasts, fuel requirements, alternatives if the flight cannot be completed, and known traffic delays ATC has advised. Fuel reserves are a separate rule, covered on the VFR fuel page.

ACS Area I, Task F is Performance and Limitations. Day 6 of the oral checklist is that task, using the POH for the airplane you will take to the test.

Common examiner follow-ups

How do I get pressure altitude?: PHAK Chapter 11 gives three methods: set the altimeter to 29.92 inHg and read it; apply the correction for the reported altimeter setting; or use a flight computer. Pressure altitude is the height above the standard datum plane, 29.92 inHg.
What is standard temperature, and what does “20 degrees above standard” mean?: PHAK Chapter 11: standard sea-level temperature is 15 °C (59 °F) and standard pressure is 29.92 inHg. Temperature decreases at about 2 °C (3.5 °F) per thousand feet up to 36,000 feet. At 5,000 feet, standard temperature is about 5 °C. “20 °C above standard” on that day is about 25 °C, not an outside air temperature of 20 °C.
Is there one formula in the PHAK, such as 120 feet per degree?: Chapter 11 of FAA-H-8083-25C does not give a feet-per-degree formula. It says to use temperature correction from tables or graphs, or a flight computer, and it works an example: pressure altitude 5,000 feet and a temperature 20 °C above standard produces a density altitude above 7,000 feet. Use the chart or the computer on the oral. Do not substitute a rule of thumb the handbook does not state.
What does high density altitude do to the airplane?: PHAK Chapter 11: as air becomes less dense it reduces power, because the engine takes in less air; thrust, because the propeller is less efficient in thin air; and lift, because the thin air exerts less force on the airfoils. The airplane performs as though it were operating at the density altitude. The handbook’s example takes a 790-foot ground roll under standard temperature at that 5,000-foot pressure altitude and says the ground run may be closer to 1,000 feet on the 20 °C-hot day.
Where does humidity fit, and what does 91.103 add?: PHAK Chapter 11 lists high elevation, low pressure, high temperature, and high humidity as conditions that result in a high density altitude. The sample chart uses pressure altitude and outside air temperature. The handbook says humidity is usually not essential to the calculation, but moist air is less dense, there is no rule of thumb or chart for that effect, and it still has to be considered. Section 91.103 requires the PIC, before any flight, to know runway lengths at airports of intended use and the takeoff and landing distance data in the approved flight manual, or other reliable performance information for aircraft that are not required to have one, using expected airport elevation, runway slope, gross weight, wind, and temperature.

Sources

Checked against the eCFR, the AIM, and FAA-H-8083-25C on October 29, 2026. The eCFR is updated continuously; the link opens the current section.