Density Altitude Explained, With Worked Examples

Density altitude is pressure altitude corrected for nonstandard temperature, and it is the altitude your airplane thinks it is at (PHAK, FAA-H-8083-25C, chapter 11). The FAA's own pamphlet on the subject puts the consequence in one sentence: if an airport at 500 ft MSL reports a density altitude of 5,000 ft, "aircraft operating to and from that airport will perform as if the airport elevation were 5,000 feet" (FAA-P-8740-2, a 2008 FAASTeam publication). The runway does not get longer to match.

The arithmetic takes two minutes and one rule of thumb. The judgment takes a POH chart and some honesty about margins.

Pressure altitude comes first

Pressure altitude is the height above the standard datum plane, the theoretical level where the pressure is 29.92 inHg, and it is what your altimeter reads with 29.92 in the Kollsman window (PHAK chapter 11). You get it one of two ways: set 29.92 and read the altimeter, or correct the field elevation for the reported altimeter setting with a table like PHAK Figure 11-3.

The correction runs roughly 1 inHg per 1,000 ft near the surface (PHAK chapter 4 gives the standard pressure lapse as approximately 1 inHg per 1,000 ft up to 10,000 ft). Three rows from the PHAK table show the direction: at 29.92 the correction is 0; at 30.10 you subtract 165 ft from field elevation; at 29.70 you add 205 ft. High pressure means a heavier air column than standard, so your pressure altitude is lower than your elevation, and the reverse. On most days the correction is a few hundred feet. It is the temperature term that does the damage.

Density altitude is pressure altitude corrected for temperature

The standard atmosphere is 15°C and 29.92 inHg at sea level, with temperature falling about 2°C per 1,000 ft (PHAK chapter 4, Figure 4-3). That table is the whole trick. Standard temperature at any pressure altitude is 15 minus 2 for every thousand feet, so standard at 5,000 ft is about 5°C and at 10,000 ft about minus 5°C (the PHAK table gives 5.1°C and minus 4.8°C).

Density altitude is "determined by first finding pressure altitude, and then correcting this altitude for nonstandard temperature variations" (PHAK chapter 4). Warmer than standard, density altitude goes up. Colder, it comes down. Under standard conditions the two are the same number, which is why a morning at 29.92 and 15°C is the only day a sea-level airport performs like the book. For every other day the PHAK gives you a chart for the correction (Figure 11-4), the POH gives you charts that skip the intermediate step, and a flight computer has a window for it.

The rule of thumb below is for the ramp, when you have a METAR on your phone and no chart in reach.

The 120 feet per degree rule of thumb

A common shortcut adds about 120 ft of density altitude for every degree Celsius the outside air temperature sits above standard. It is a rule of thumb. It appears in neither the PHAK nor the pamphlet; it is a straight-line approximation of the standard-atmosphere chart that pilots pass around, so before trusting it, check it against the FAA's own published numbers. That is what the table does.

The published figures are from PHAK FAA-H-8083-25C chapter 11 and the Density Altitude Rule-of-Thumb Chart in FAA-P-8740-2; the right-hand column is our arithmetic. Three cases, three answers within 250 ft. The shortcut errs high each time, which is the safe direction for a planning estimate.

Use it to decide whether the day is worth a closer look, never as the number you take to the runway.

A worked example at sea level, KFXE on a 35°C afternoon

Suppose you are departing Fort Lauderdale Executive (KFXE), field elevation 13 ft (FAA approach chart AL-5942, October 2026 cycle), at 3 p.m. in July. The ASOS gives 35°C and an altimeter of 29.92.

1. Pressure altitude. Altimeter 29.92, correction 0. Pressure altitude is the field elevation, 13 ft. Call it sea level.
2. Standard temperature. At sea level, 15°C.
3. Deviation. 35 minus 15 is 20°C above standard.
4. Density altitude. 20 times 120 is 2,400 ft. The pamphlet's chart brackets the answer between 1,900 ft at 90°F and 2,500 ft at 100°F, so call it 2,200 to 2,400 ft.

A sea-level airport is performing like a field at 2,400 ft. The PHAK's example takeoff table (Figure 11-24, a 2,400 lb airplane with flaps 10°) shows what that costs. At sea level the ground roll is 795 ft on a 0°C day, 995 ft at 30°C and 1,065 ft at 40°C, and the distance to clear a 50 ft obstacle grows from 1,460 ft to 1,810 ft and then 1,945 ft. On KFXE's 6,002 ft main runway, 1,945 ft leaves room to spare. Put the same 1,945 ft on a 2,500 ft grass strip in August, with the pamphlet's warning that long grass "can easily double your takeoff distance," and you are looking at the accident it says happens "in less time than it takes to tell about it."

A worked example at Leadville, KLXV

Now the other end. Lake County Airport at Leadville, Colorado (KLXV) has a field elevation of 9,934 ft (FAA approach chart AL-9146, October 2026 cycle) and a single 6,400 ft runway. Suppose a midsummer afternoon: 25°C, altimeter 30.10.

1. Pressure altitude. From the PHAK table, 30.10 means subtract 165 ft. 9,934 minus 165 is 9,769 ft.
2. Standard temperature. 15 minus 2 times 9.8 is about minus 4.6°C. Call it minus 5°C.
3. Deviation. 25 minus (minus 5) is 30°C above standard.
4. Density altitude. 30 times 120 is 3,600 ft. Add it to 9,769 and you get about 13,400 ft. A chart or calculator will come in a few hundred feet lower, and either answer is well above the 8,000 ft top row of the PHAK's example takeoff chart.

Twenty-five degrees is not a heat wave. It is a pleasant afternoon. That is the point of the example. At a high-elevation field, ordinary temperatures produce density altitudes that most POH takeoff charts do not print rows for, and the pamphlet's advice to schedule operations "during the cool hours of the day (early morning or late afternoon)" stops being optional.

What the POH chart does that the rule of thumb cannot

Density altitude is an input. The output you fly is a distance in feet and a climb rate in feet per minute, and only the airplane's own performance data can turn one into the other. The PHAK's illustration: an airplane at 5,000 ft pressure altitude needs 790 ft of ground run under standard temperature, but with the temperature 20°C above standard "the density level is above 7,000 feet, and the ground run may be closer to 1,000 feet" (PHAK chapter 11).

Look at the shape of the Figure 11-24 table again. Ground roll is 795 ft at sea level and 0°C, 1,500 ft at 5,000 ft and 20°C, and 2,060 ft at 8,000 ft and 20°C. The distance to clear a 50 ft obstacle rises from 1,460 ft to 4,480 ft over the same range. The 8,000 ft row at 30°C and 40°C holds dashes. The chart runs out before the weather does. When your conditions fall off the end of the table, that is the manufacturer declining to make a promise, and the right response is not interpolation.

If the POH is not in reach, the pamphlet offers the Koch chart as an approximation and gives one reading from it. At 100°F and 6,000 ft pressure altitude, add 230 percent to the sea-level takeoff distance and expect the climb rate to fall 76 percent, so 1,000 ft to clear 50 ft becomes 3,300 ft and 500 fpm becomes 120 fpm. The pamphlet labels these "typical representative values" and tells you to consult the AFM or POH for exact ones. Treat the Koch chart the same way as the 120 ft rule: good enough to say no, never good enough to say yes.

Humidity's smaller effect

Water vapor is lighter than dry air, so humid air is less dense, and the PHAK is careful about the size of the effect: humidity "is usually not considered an important factor in calculating density altitude and aircraft performance, but it is a contributing factor" (PHAK chapter 4). Its worked example at a pressure altitude of 8,000 ft, 80°F and a dew point of 75°F gives a density altitude of 11,564 ft, and "with no humidity, the density altitude would be almost 500 feet lower."

Five hundred feet is not nothing, and it is not the headline. The pamphlet's advice is practical: "if high humidity does exist, however, it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate" (FAA-P-8740-2). The mechanism it names is engine power, not aerodynamics. Our opinion, not the pamphlet's: a 10 percent pad on any chart distance is a sensible floor on a hot day, humid or not.

What to check before engine start

Density altitude is a before-engine-start problem because every lever you have works on the ground. Run it in this order.

1. Compute the number. Field elevation, altimeter setting, temperature. Pressure altitude first, then density altitude, by the rule of thumb for a first look and by the POH or a flight computer for the number you plan on. FlyFren's airport pages show field elevation alongside the current NWS Aviation Weather Center observation, for supplemental use; the official source is the ASOS or AWOS on the field.
2. Open the POH takeoff chart at your weight. Read ground roll and distance over a 50 ft obstacle at the density altitude, or at pressure altitude and temperature, whichever axes the chart uses. If your conditions are off the edge of the chart, stop here.
3. Compare with the runway. Runway length is in the Chart Supplement. Subtract the chart distance and look hard at what is left. Add the pamphlet's 10 percent for humidity, and more for grass, soft ground or a tailwind; its figure for long grass, sand, mud or deep snow is double.
4. Check the climb against the terrain. A 76 percent cut in climb rate turns a 500 fpm airplane into a 120 fpm one. Work out what gradient the terrain off the departure end demands and whether that climb rate, at your groundspeed, delivers it. The temperature aloft, which the winds aloft forecast gives you, says whether the density altitude stays high all the way up. At Leadville the question is not whether you will clear the fence.
5. Lean for takeoff if the engine is normally aspirated. The pamphlet says that at density altitudes above 5,000 ft, leaning a normally aspirated engine for maximum power is "essential," unless it has automatic altitude mixture control; turbocharged engines need not be leaned.
6. Change what you can. Fuel and passengers are the weight you control. The clock is the temperature you control: the pamphlet's "cool hours of the day" are often the difference between a chart row that exists and one that does not.

If any step produces a number you would not accept from someone else, the airplane does not care that it is you. Wait for the morning.

This article is educational and is not a substitute for the current regulations, the AIM, official weather briefings, or instruction from a CFI. Check the current text at the linked sources before you rely on any detail.