Updated October 2026
Pressure altitude is what your altimeter reads with 29.92 inches of mercury set in the window, and the shortcut formula is field elevation plus (29.92 minus the altimeter setting) times 1,000. Density altitude is pressure altitude corrected for nonstandard temperature, the altitude the airplane performs as if it were at. At Van Nuys, 802 feet up, a 105°F afternoon puts it near 4,000 feet.
Both numbers sound like ground-school trivia until the first hot afternoon when a trainer near gross weight uses most of the runway and then climbs as if it were tired. Pressure altitude is the input. Density altitude is the answer, and it tells you how the engine, the propeller and the wing are going to behave today rather than on the day the handbook was written.
Below are both formulas with worked examples from our own field, a calculator, and what the numbers mean in the airplanes we fly every day.
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The Five Altitudes A Pilot Uses (Indicated, Pressure, Density, True, Absolute)
The FAA's Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C, chapter 8) names five kinds of altitude. Only two of them, true and absolute altitude, are tied to the earth. The other three are measured against air pressure, which is why they move around with the weather.
| Altitude | What It Means | Where You Use It |
|---|---|---|
| Indicated | What the altimeter shows with the current local altimeter setting in the window | Flying the pattern, holding an assigned altitude below 18,000 ft |
| True | Actual height above mean sea level | Charted elevations of airports, terrain and obstacles |
| Absolute | Height above the ground directly below (AGL) | Minimum safe altitudes, ground reference maneuvers |
| Pressure | Height above the standard datum plane, where pressure is 29.92 inHg | Performance charts, flight levels from 18,000 ft up |
| Density | Pressure altitude corrected for nonstandard temperature | Takeoff distance, climb rate, engine power |
Indicated and true altitude agree only when the air is standard. In colder air the airplane is lower than the altimeter says; the handbook's memory aid is "from hot to cold, look out below." For performance, the two that matter are pressure and density altitude.
Pressure Altitude: Definition, Formula And A Worked Example
Pressure altitude is your height above the standard datum plane, an imaginary level where the air pressure is 29.92 inches of mercury (1013.2 millibars). On a day when sea-level pressure is exactly 29.92, pressure altitude and field elevation are the same number. On every other day they differ, by roughly 1,000 feet for each inch of mercury.
Pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000
Work it once for Van Nuys. The field sits at 802 feet. Say the ATIS gives an altimeter setting of 29.82. Subtract: 29.92 minus 29.82 is 0.10. Multiply by 1,000 and you have 100 feet. Add that to 802 and the pressure altitude is 902 feet.
Now a cold, clear morning after a winter front, altimeter 30.12. This time 29.92 minus 30.12 is −0.20, so you take 200 feet away: pressure altitude 602 feet, lower than the field itself. High pressure makes the air act as if you were lower. Low pressure does the opposite.
The sign is where students slip. A setting above 29.92 gives a pressure altitude below field elevation. If your answer moves the wrong way, check the subtraction before anything else.
How To Read Pressure Altitude Off The Altimeter (29.92)
Sitting in the airplane, you can skip the formula. Turn the knob until the setting window (the Kollsman window on a steam-gauge altimeter) reads 29.92, and read the altimeter. That is pressure altitude straight off the instrument, and it is the first method the handbook gives. Then dial the local setting back in before you taxi, because indicated altitude is what you fly by.
Above 18,000 feet MSL everybody flies this way. 14 CFR 91.121 requires 29.92 at or above 18,000 feet, and below that the current reported setting of a station along the route within 100 nautical miles. The altitudes up there are flight levels, which 14 CFR 1.1 defines as levels of constant pressure related to 29.92. FL250, the ceiling of our Cirrus SR22 Turbo N422SN, is a pressure altitude of 25,000 feet.
Two habits from our ramp. Do the 29.92 check before the run-up, not while holding short, so the number is on your kneeboard before you need it. And keep the other direction in mind. When the pressure falls during a flight and you do not update the setting, the altimeter reads higher than you really are. "High to low, look out below" is the handbook's phrase, and over the ridges north of the Valley it deserves to be taken literally.
Standard Atmosphere, Lapse Rate And ISA Deviation
Every performance chart in every POH is built on the International Standard Atmosphere, a model day that never quite happens. At sea level it is 15°C (59°F) and 29.92 inHg. Temperature falls about 2°C per 1,000 feet up to 36,000 feet, and pressure falls about one inch of mercury per 1,000 feet in the lower atmosphere (PHAK, chapter 11).
From those two rates you can find the standard temperature anywhere: start at 15 and take away 2 for every thousand feet. At Van Nuys that is 15 minus 1.6, about 13°C. ISA deviation is the actual temperature minus that standard figure. An August afternoon at 35°C is ISA +22. A January morning at 8°C is ISA −5.
| Altitude | Standard Temperature | Standard Pressure (Approx.) |
|---|---|---|
| Sea level | 15°C (59°F) | 29.92 inHg |
| 802 ft (Van Nuys) | 13.4°C (56°F) | 29.07 inHg |
| 2,000 ft | 11°C (52°F) | 27.82 inHg |
| 5,000 ft | 5°C (41°F) | 24.90 inHg |
| 6,752 ft (Big Bear City) | 1.5°C (35°F) | 23.37 inHg |
| 10,000 ft | −5°C (23°F) | 20.58 inHg |
Jet performance data and international weather products write temperatures as deviations ("ISA +15") all the time. Get used to thinking that way early. The deviation, not the raw temperature, is what drives density altitude.
Density Altitude: Definition, Formula And The Koch Chart
Density altitude is pressure altitude corrected for nonstandard temperature. Put another way, it is the altitude in the standard atmosphere where the air would be as thin as the air you are actually sitting in. Everything that makes a trainer fly responds to density, and nothing on it can read the field elevation sign.
Most instructors teach one rule of thumb. Our mountain flying checkout uses it for Big Bear:
Density altitude ≈ pressure altitude + 120 × (OAT − standard temperature), with both temperatures in °C.
Each degree Celsius above standard adds about 120 feet. It is an approximation, and on hot days it comes out slightly high. That is the safe direction to be wrong in.
Humidity pushes the same way, because moist air is lighter than dry air. The FAA's density altitude pamphlet (FAA-P-8740-2) treats it as a smaller effect, felt mostly as lost engine power. Its advice is to add 10 percent to the computed takeoff distance when the air is humid.
The Koch chart is the backup for when you have no handbook. Lay a straight edge from the outside air temperature to the airport's pressure altitude, and read off the percentage added to takeoff distance and the percentage taken off the climb rate. The pamphlet's example is 100°F at 6,000 feet pressure altitude. That adds 230 percent to the takeoff distance, so a 1,000-foot roll becomes about 3,300 feet. It also takes 76 percent off the climb, so 500 feet per minute becomes about 120. The FAA is plain that the POH comes first and the Koch chart second. So are we.
Density Altitude Calculator (Field Elevation, Altimeter Setting, Temperature)
The calculator runs three steps. First, pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000. Second, standard temperature = 15 − 2 × (pressure altitude ÷ 1,000), and ISA deviation = outside air temperature − standard temperature. Third, density altitude ≈ pressure altitude + 120 × ISA deviation. Worked example for Van Nuys at 105°F (40.6°C) with an altimeter setting of 29.82. Pressure altitude is 802 + 100 = 902 feet. Standard temperature is 15 − 1.8 = 13.2°C, so the ISA deviation is +27.4°C. Density altitude is 902 + (120 × 27.4), about 4,190 feet. The exact standard-atmosphere equations for dry air put the same day nearer 3,950 feet, so call it 4,000 and plan for the higher figure.
Take the altimeter setting from the ATIS or the METAR. What comes out is a planning number; the takeoff and climb figures you fly by still come from Section 5 of the POH at the airplane's actual weight. If you prefer the slide rule, our E6B flight computer guide works the same problem on the wheel.
What A 105°F Day At Van Nuys Does To Takeoff Distance And Climb
The San Fernando Valley gets hot. NOAA normals put the average August high at Van Nuys at 93.1°F, and a heat wave adds ten degrees to that without much effort. The field elevation never changes. The airplane's idea of it does.
| Day | Altimeter | Temperature | Pressure Altitude | Density Altitude (Rule Of Thumb) |
|---|---|---|---|---|
| Van Nuys, winter morning after a front | 30.15 | 10°C (50°F) | 572 ft | About 100 ft |
| Van Nuys, standard day | 29.92 | 13°C (56°F) | 802 ft | About 800 ft |
| Van Nuys, August afternoon | 29.92 | 35°C (95°F) | 802 ft | About 3,400 ft |
| Van Nuys, heat-wave afternoon | 29.82 | 40.6°C (105°F) | 902 ft | About 4,200 ft |
| Big Bear City, summer afternoon | 29.92 | 30°C (86°F) | 6,752 ft | About 10,200 ft |
At about 4,000 feet density altitude the air around the airplane is roughly 11 percent thinner than standard sea-level air, and you feel it three ways on the same takeoff.
The engine makes less power. The 160-horsepower Lycoming in our Warriors, N3034Q and N6312C, has no turbocharger, so thinner air means less air and less fuel burned on every stroke. The propeller takes a smaller bite. And the wing needs a higher true airspeed for the same lift. You still lift off at the same indicated airspeed, but your true airspeed, and your groundspeed in calm wind, is about 6 percent higher. Slower acceleration toward a higher speed over the ground means a longer roll. How much longer, only the chart can say.
The climb suffers most. Power available drops and power required rises, so the surplus that makes an airplane climb gets squeezed from both ends. The Airplane Flying Handbook warns that at high density altitude or maximum weight an airplane may lift off and then be unable to climb out of ground effect. The Santa Susana ridge northwest of the field does not move out of the way for anyone.
What we do about it at Van Nuys is not complicated. Before a summer flight we run the POH numbers for the real weight, not a guess. 14 CFR 91.103(b) makes takeoff and landing distance information part of every preflight anyway. Our trainers normally use 16L/34R, which is 4,000 feet long; the 8,001-foot 16R/34L is there when the tower assigns it, but extra concrete does nothing for a weak climb once the wheels are off. If you can pick your time, fly early. Discovery flights run from 10 AM to 7 PM, and in August a 10 AM slot gets cooler, smoother air than 3 PM. Our page on flyable days in Los Angeles covers the year month by month.
Common Written-Test And Checkride Questions
Density altitude is not optional knowledge. 14 CFR 61.105(b)(8) names its effects on takeoff and climb performance as a required subject for private pilot applicants. The Private Pilot ACS (FAA-S-ACS-6C) tests it under Performance and Limitations, element PA.I.F.K2a. The questions we hear most, with the answers examiners want:
- "How do you find pressure altitude in the airplane?" Set 29.92 in the window and read the altimeter, then reset the local setting.
- "Field elevation 802, altimeter 30.12. What is the pressure altitude?" 602 feet. The setting is above 29.92, so the answer is below field elevation.
- "Will you lift off at a higher indicated airspeed on a hot day?" No. Same indicated airspeed, higher true airspeed and groundspeed, so a longer roll.
- "Which hurts performance: high, hot or humid?" All three lower the density. Altitude and temperature do most of it; humidity mainly costs engine power.
- "What do you set at 18,000 feet?" 29.92, under 14 CFR 91.121, and from there up you fly flight levels.
For the rest of the knowledge test, see our private pilot written exam guide. Examiners often pull from here to the next thread, why maneuvering speed changes with weight. Both are taught on the ground and in the air in our private pilot course in Los Angeles.
Related Reading
- E6B flight computer, with a true airspeed calculator that uses pressure altitude and temperature.
- Maneuvering speed explained, the other number that changes on the day.
- Short-field and soft-field takeoffs and landings, for when the runway and the air are both tight.
- Private pilot course brochure, the syllabus where performance planning is taught.
- Mountain flying checkout, Big Bear and Mammoth worked with the same arithmetic.
- Aerodynamics for student pilots, the hub for lift, drag and the spoke pages.
Pressure Altitude And Density Altitude FAQ
What Is The Formula For Pressure Altitude?
Pressure altitude equals field elevation plus 29.92 minus the altimeter setting, times 1,000. At Van Nuys, elevation 802 feet, with a setting of 30.12, that is 802 minus 200, or 602 feet.
How Do You Calculate Density Altitude Without A Calculator?
Find the pressure altitude, work out the standard temperature for it (15°C minus 2°C per 1,000 feet), and add about 120 feet for every degree Celsius the actual temperature is above standard. At Van Nuys on a 95°F day that gives roughly 3,400 feet.
Is Density Altitude Higher On Hot Days?
Yes. Warm air is less dense, so density altitude rises above pressure altitude when the temperature is above standard and falls below it when the temperature is below standard. On a cold winter morning with high pressure, density altitude at Van Nuys can be close to 100 feet.
Does Humidity Affect Density Altitude?
Yes, a little. Moist air is lighter than dry air, so humidity raises density altitude and mostly costs engine power. The FAA density altitude pamphlet suggests adding 10 percent to the computed takeoff distance when it is humid.
Why Does An Airplane Need More Runway When Density Altitude Is High?
The engine and propeller produce less thrust in thin air, and the wing needs a higher true airspeed for the same lift. The airplane lifts off at the same indicated airspeed, but it accelerates more slowly toward a higher groundspeed, so the roll is longer and the climb is weaker.
Do I Set 29.92 On The Altimeter For Every Flight?
No. Below 18,000 feet MSL you fly with the current local altimeter setting, as 14 CFR 91.121 requires. You set 29.92 briefly on the ground to read pressure altitude, and permanently only at or above 18,000 feet.
Feel Density Altitude From The Left Seat
The numbers make more sense once you have watched a climb rate change between a December morning and an August afternoon. A discovery flight is $229 for 60 minutes in a Piper Warrior from Van Nuys. You fly it from the left seat, and it is credited in full toward training if you enroll within 24 hours. If you want the whole plan first, including the $10,400 Standard private pilot package with $2,500 down, book a free consultation and an instructor will walk you through it.
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