Density Altitude Calculator and Guide: What It Is and How to Calculate It

The FAA's density altitude pamphlet opens with a warning: "Hot, high, and humid weather conditions can cause a routine takeoff or landing to become an accident in less time than it takes to tell about it." [3, p. 3] This page explains what density altitude is, how to calculate it by hand and with the National Weather Service formula, what it does to takeoff and climb, and how to use the Koch chart. It draws on the FAA Pilot's Handbook of Aeronautical Knowledge (PHAK), the Instrument Flying Handbook, the Aviation Weather Handbook, FAA pamphlet FAA‑P‑8740‑2, NTSB safety alerts, and the NWS calculator formulas [1][2][3][4][5].

What is density altitude?

"Density altitude is pressure altitude corrected for nonstandard temperature." [1, p. 4-4] The Instrument Flying Handbook puts it another way: "Density altitude, then, is the vertical distance above sea level in the standard atmosphere at which a given density is to be found." [2, p. 4-5] In plain terms, it is the altitude your airplane thinks it is at.

The reference is the standard atmosphere: 15 °C and 29.92 inches of mercury at sea level [1, p. 4-3]. "A standard temperature lapse rate is one in which the temperature decreases at the rate of approximately 3.5 °F or 2 °C per thousand feet up to 36,000 feet." [1, p. 11-2] Pressure altitude is the altitude on an altimeter set to 29.92 [1, p. 4-4]. On a standard day the numbers line up, but as the Aviation Weather Handbook says, "Density altitude equals field elevation during standard atmospheric conditions, but conditions are rarely standard." [4, para. 8.4.1.5]

What raises density altitude?

"The conditions that result in a high density altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some combination of these factors." [1, p. 4-4] Here is how each one works:

Source conflict, noted: the FAA documents agree humidity raises density altitude but differ on how much it matters. The Aviation Weather Handbook says dewpoint "is also a contributing factor, but its effects are generally negligible." [4, para. 8.4.1.5] The PHAK calls humidity a contributing factor and shows an example where it added almost 500 ft [1, p. 4-5]. The FAA pamphlet ties humidity mainly to engine power and advises adding 10 percent to computed takeoff distance when it is high [3, p. 4]. This page uses the more conservative advice: account for humidity on hot, humid days.

How do you calculate density altitude?

By hand, in three steps. Each step has an FAA source; the arithmetic is simple enough to do on a kneeboard.

Step 1: find pressure altitude

The PHAK gives three ways: set the altimeter to 29.92 and read it, apply a correction for the reported altimeter setting, or use a flight computer [1, p. 11-3]. For the correction, the PHAK's altimeter table has the values (30.10 inHg means subtract 165 ft), and the standard pressure lapse rate of about 1 inch of mercury per 1,000 ft is a quick check [1, pp. 11-2 and 11-3]. A low altimeter setting raises pressure altitude; a high one lowers it.

Step 2: find the standard temperature at that altitude

Start at 15 °C and subtract 2 °C per 1,000 ft of pressure altitude [1, p. 11-2]. At 5,000 ft, standard is 5 °C. Compare that with the reported temperature from the METAR or ATIS.

Step 3: add 120 ft per degree above standard

"If a chart is not available, the density altitude can be estimated by adding 120 feet for every degree Celsius above the ISA." [2, p. 4-6] The Handbook's own example: at 3,000 ft pressure altitude, standard is 9 °C. At 20 °C the air is 11 °C warmer, and 11 × 120 = 1,320 ft, for a density altitude of 4,320 ft [2, p. 4-6]. Colder than standard works the same way in reverse: subtract.

Charts, flight computers, and humidity

For a closer answer, the Handbook points to a Koch chart or a flight computer with a density altitude function [2, pp. 4-5 and 4-6], and the PHAK has a density altitude chart that reads pressure altitude and temperature [1, p. 11-4]. None of these covers humidity. "There is no rule-of-thumb or chart used to compute the effects of humidity on density altitude" [1, p. 11-5], which is why the PHAK sends pilots to the NWS online calculator [1, p. 4-5].

Worked example: a hot afternoon at a 6,000 ft field

An illustrative case, not a real airport: field elevation 6,000 ft, altimeter 30.10 inHg, temperature 32 °C, dewpoint 10 °C.

The methods land within about 250 ft of each other, which is about what a 2 °C error in the temperature would change anyway. That is the honest precision of any density altitude number. On this afternoon the airplane performs as if it were taking off from a field above 9,000 ft. For the takeoff, enter your POH chart with 5,835 ft pressure altitude, 32 °C, and your actual weight; most takeoff charts use pressure altitude and temperature directly, and the PHAK says "Proper accounting of pressure altitude and temperature is mandatory for accurate prediction of takeoff roll distance." [1, p. 11-16]

Density altitude calculator

Enter field elevation, the altimeter setting, and temperature, plus dewpoint if you have it. The calculator converts the altimeter setting to station pressure, then applies the NWS formulas below, and shows the result rounded to the nearest 50 ft with the IFH rule of thumb beside it as a cross-check [5][2, p. 4-6]. It runs in your browser and sends nothing anywhere. The answer describes the air, not your airplane: the takeoff and landing charts in your AFM or POH govern.

What formula does the calculator use?

The formulas published with the NWS El Paso forecast office's density altitude calculator, the calculator the PHAK refers pilots to [5][1, p. 4-5]. The NWS page asks for station pressure: "Enter the actual station pressure (not the altimeter setting)" [5]. An altimeter setting is corrected to sea level, so step 1 converts it back:

1. Station pressure from the altimeter setting A (inHg) and field elevation h (meters): Pstn = A × ((288 - 0.0065 × h) / 288) ^ 5.2561
2. Pressure altitude (ft) from station pressure in millibars, where 1 inHg = 33.8639 mb: PA = (1 - (Pmb / 1013.25) ^ 0.190284) × 145,366.45
3. Vapor pressure (mb) from dewpoint Td (°C): e = 6.11 × 10 ^ (7.5 × Td / (237.3 + Td))
4. Virtual temperature (K) from air temperature T (K): Tv = T / (1 - (e / Pmb) × (1 - 0.622)). With no dewpoint, Tv = T.
5. Density altitude (ft), with Tv in degrees Rankine (K × 1.8): DA = 145,366 × (1 - (17.326 × Pstn / Tv) ^ 0.235)

Checked against the PHAK: for its example at 8,000 ft (22.22 inHg station pressure, 80 °F, dewpoint 75 °F) these formulas give the PHAK's 11,564 ft [1, p. 4-5]. For the PHAK's 5,048 ft field at 30 °C they give about 7,875 ft against the PHAK's 7,855 ft [1, p. 8-7]. Differences of that size are rounding in the constants, not errors worth worrying about.

What does high density altitude do to performance?

Thin air hits the airplane three ways at once. The PHAK lists the losses [1, p. 4-4]:

• "Power because the engine takes in less air"
• "Thrust because a propeller is less efficient in thin air"
• "Lift because the thin air exerts less force on the airfoils"

Takeoff roll

"An increase in density altitude can produce a twofold effect on takeoff performance:" a greater takeoff speed, and less thrust with a reduced net accelerating force [1, p. 11-16]. You lift off at the same indicated airspeed, but the true airspeed and groundspeed are higher, so the roll is longer [4, para. 8.4.1.5]. The PHAK's example: at 5,000 ft pressure altitude, a ground run of 790 ft at standard temperature may be closer to 1,000 ft when the air is 20 °C warmer than standard [1, p. 11-4]. Surface makes it worse; the FAA pamphlet warns that "long grass, sand, mud, or deep snow can easily double your takeoff distance." [3, p. 6]

Climb

"An increase in altitude also increases the power required and decreases the power available. Therefore, the climb performance of an aircraft diminishes with altitude." [1, p. 11-8] At the same indicated airspeed you cover more ground per minute, so the climb angle is shallower as well [4, para. 8.4.1.5]. That is what matters with rising terrain or trees off the end of the runway.

Approach and landing

Approach at the normal indicated airspeed; the true airspeed will be higher and the landing roll longer [3, p. 4]. The PHAK puts a number on it: "The minimum landing distance at 5,000 feet is 16 percent greater than the minimum landing distance at sea level." [1, p. 11-17]

The Koch chart

When the AFM or POH is not available, the FAA pamphlet says to use the Koch chart [3, p. 4]. "To find the effect of altitude and temperature, connect the temperature and airport altitude by a straight line." Where the line crosses the middle scales, read the percentage to add to takeoff distance and to subtract from rate of climb [3, p. 5]. The pamphlet's example is 100 °F at 6,000 ft pressure altitude: add 230 percent. "Therefore, if your standard temperature sea level takeoff distance normally requires 1,000 feet of runway to climb to 50 feet, it would become 3,300 feet under the conditions shown in the chart." "In addition, the rate of climb would be decreased by 76 percent." A 500 fpm climb becomes 120 fpm [3, p. 6]. The same chart is in the Instrument Flying Handbook as Figure 4-7 [2, p. 4-6]. It shows typical values for light airplanes, not your airplane: "For exact values, consult your AFM/ POH." [3, p. 6]

Why is density altitude a safety issue?

Because the airplane can be legal, loaded within limits, and still unable to climb. The NTSB, in its safety alert on weight and balance: "Even if an aircraft is under or near its maximum gross takeoff limit, atmospheric conditions can degrade the aircraft's performance enough to prevent it from attaining or maintaining a climb." [7, p. 1] The same alert counts "Between 2008 and 2016, the probable causes of 136 general aviation (GA) accidents were related to pilots improperly conducting preflight performance calculations for weight and balance or not conducting them at all." and adds that "One-third of these accidents resulted in pilot and/or passenger deaths." [7, p. 1]

Mountain flying concentrates the risk. The NTSB warns that "Pilots with limited or no training in mountain flying can be surprised about their aircraft's different performance at high density altitude, often leading to serious or fatal accidents." [6, p. 1] In one fatal accident it describes, a Piper Cherokee 235 trying to cross a high mountain pass, the NTSB found that in the conditions at the time the airplane's "climb rate would have been reduced by more than 90 percent." [6, p. 1]

The rule behind the habit is 14 CFR 91.103: before a flight, the pilot in command must become familiar with runway lengths and takeoff and landing distance data "relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature." [8, para. (b)]

How do you fly safely on a high density altitude day?

• Run the numbers for the real conditions. Use the POH takeoff, climb, and landing data for the pressure altitude, temperature, weight, wind, and surface you will actually have [1, p. 11-16][8]. "Be prepared and conduct takeoff and landing distance calculations as part of your preflight planning." [7, p. 3]
• Listen for it. ASOS and AWOS broadcasts include density altitude when it is more than 1,000 ft above field elevation, and ATIS may say "check density altitude" [4, para. 3.4.2.24].
• Reduce weight. High density altitude "may require a reduction in weight before flight is attempted." [1, p. 10-6]
• Fly in the cool hours. The FAA pamphlet suggests early morning or late afternoon when performance is in question [3, p. 4].
• Lean as the POH says. The pamphlet calls leaning normally aspirated engines for maximum power on takeoff essential above 5,000 ft density altitude, unless the airplane has automatic mixture control [3, p. 4].
• Add margin for humidity. On humid days, add 10 percent to computed takeoff distance and expect a weaker climb [3, p. 4].
• Get mountain training. The NTSB says "Flight instructors should encourage their students to attend a quality mountain flying course before attempting flight in mountainous terrain or at high density altitudes." [6, p. 2]

Where are the official density altitude references?

Official density altitude references

• Start here: PHAK Chapter 4, Density Altitude (PDF pages 91 to 92) and Chapter 11, Aircraft Performance (PDF pages 258 to 276) [1].
• Instrument Flying Handbook, Chapter 4, density altitude and the Koch chart, Figure 4-7 (PDF pages 82 to 83) [2].
• FAA pamphlet FAA‑P‑8740‑2, Density Altitude, with the rule-of-thumb and Koch charts [3].
• Aviation Weather Handbook, Section 8.4.1.5 (PDF page 108) and Appendix C, Density Altitude Computation Chart (PDF page 486) [4].
• NWS El Paso density altitude calculator and its formula notes [5].

Links checked September 30, 2026: all returned HTTP 200 or 206 (partial PDF response).

Where does density altitude fit in a disciplined decision?

PlaneWX is decision support for general aviation pilots, built around one idea: "Fly like it's your job." Treat the performance question the way a professional crew would, with the same process every time. That process is a loop: Brief, then FRAT, then Fly or Stay, then Debrief, with a Mentor if you want one. Density altitude belongs in the Brief and the FRAT. Here is how PlaneWX handles it, as described in the help center [10]:

• Brief: a caution, not a score. "Density altitude alone does not deduct WX Score points." When density altitude is elevated at departure or arrival, a red (high or critical) or yellow (moderate) caution appears above the WX Score dial, and there is "No banner when DA is fine" (Density Altitude Caution).
• Brief: inputs you can check. Within 6 hours of departure it uses the METAR temperature, dewpoint, and altimeter; further out, the NWS hourly forecast temperature and dewpoint with the latest METAR altimeter. The Terminal Weather cards show the inputs "so you can punch the same numbers into your own DA calculator or POH performance tables." (Density Altitude Caution)
• Brief: what it is not. "Not runway-length math for every aircraft. Most types get the environmental caution only. Verify takeoff and landing distance in your own POH." Types with runway performance math, today the TBM 900, can lose WX Score points when the POH distance exceeds the runway (Density Altitude Caution).
• FRAT. The flight risk assessment opens within 4 hours of departure. It can show an OK or short runway note as a hint, and "PIC still runs performance (weight and balance / AFM)." (FRAT) A FRAT is the FAA's form for recording hazards like this one [9, p. 3-10].
• Fly or Stay. "PlaneWX never recommends GO or NO-GO. You make the call as PIC." (The Risk Loop)
• Debrief. If the takeoff used more runway than you planned, write it down while it is fresh. Self Debrief is a PlaneWX Labs feature for Pro Plus (The Risk Loop).

The help center is plain about the limits: "PlaneWX is not a substitute for a complete, independent pre-flight weather briefing. Pull one from a source you already trust before you fly." (The Risk Loop) For the rest of the risk picture, see the flight risk assessment tool guide and the METAR guide for where the temperature and altimeter come from.

See density altitude for your next departure and arrival in a PlaneWX briefing, or read how PlaneWX shows density altitude.

Frequently asked questions

What is density altitude?

Density altitude is pressure altitude corrected for nonstandard temperature. "Regardless of the actual altitude of the aircraft, it will perform as though it were operating at an altitude equal to the existing density altitude." [1, p. 4-4]

How do you calculate density altitude?

Find pressure altitude (set 29.92 on the altimeter, or correct field elevation for the altimeter setting), work out the standard temperature for that altitude (15 °C minus 2 °C per 1,000 ft), then add about 120 ft for every degree Celsius the air is warmer than standard [1, p. 11-3][2, p. 4-6]. A flight computer, a density altitude chart, or the NWS formula gives a closer answer [5].

What is the 120 in the density altitude formula?

It is the FAA Instrument Flying Handbook rule of thumb: "If a chart is not available, the density altitude can be estimated by adding 120 feet for every degree Celsius above the ISA." [2, p. 4-6] It is an estimate, not the exact physics, and it ignores humidity.

What effect does high density altitude have on aircraft performance?

It reduces engine power, propeller thrust, and wing lift [1, p. 4-4]. The FAA lists the results as increased takeoff distance, reduced rate of climb, higher true airspeed on approach and landing at the same indicated airspeed, and a longer landing roll [3, p. 4].

Does humidity affect density altitude?

Yes, a little. Moist air is less dense than dry air, so humidity raises density altitude. In a PHAK example at 8,000 ft and 80 degrees, humidity added almost 500 ft [1, p. 4-5]. The FAA density altitude pamphlet advises adding 10 percent to computed takeoff distance when humidity is high [3, p. 4].

Is density altitude the same as pressure altitude?

Only at standard temperature. Pressure altitude is the altitude on an altimeter set to 29.92. Density altitude corrects it for temperature, so on a hot day density altitude is higher than pressure altitude [1, p. 4-4][4, para. 8.4.1.5].

What is a Koch chart?

An FAA chart that estimates the percentage to add to takeoff distance and to subtract from climb rate for a given temperature and airport pressure altitude. The FAA says to use it when the AFM or POH is not available, and that it shows typical values, not exact ones [3, pp. 4 and 6].

Can I use a density altitude calculator instead of my POH?

No. A calculator tells you how thin the air is. Only the takeoff and landing performance data for your airplane tell you whether the runway and climb gradient are enough. Federal rules require you to know that data before the flight [8].

Does PlaneWX calculate density altitude?

Yes. The help center says PlaneWX shows density altitude at departure and arrival, with the temperature and altimeter inputs it used, and a red or yellow caution when it is elevated. It does not deduct WX Score points for density altitude alone, and it does not replace your POH performance check [10].

Glossary

Every term below links to the FAA, NWS, or NOAA document that defines it. The same definitions appear when you hover, focus, or tap a dotted term on this page.

AFM (Airplane Flight Manual)

The manufacturer’s FAA-approved document for a specific make and model, containing its operating procedures and limitations. Source: FAA Pilot’s Handbook of Aeronautical Knowledge, Glossary

Altimeter group

Coded as A plus four digits in inches of mercury without the decimal point, so A2992 is an altimeter setting of 29.92 inHg. Source: FAA Aviation Weather Handbook, 24.4.3.11

ASOS (Automated Surface Observing System)

The automated weather station network that is the nation’s primary source of surface observations. Source: FAA Aviation Weather Handbook, 24.3.1

AWOS (Automated Weather Observing System)

An automated weather station similar to ASOS that generally reports fewer elements. Source: FAA Aviation Weather Handbook, 24.3.2

Density altitude

Pressure altitude corrected for nonstandard temperature. An airplane performs as though it were at this altitude, whatever the field elevation. Source: FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 4, Density Altitude

Dewpoint

The temperature to which air must be cooled, at constant pressure and moisture content, to become saturated. Source: FAA Aviation Weather Handbook, 24.4.3.10

FRAT (Flight Risk Assessment Tool)

A form or checklist for recording flight hazards and the risk they add up to before you fly. Source: FAA Risk Management Handbook, Chapter 3, Using a Flight Risk Assessment Tool (FRAT)

ISA (International Standard Atmosphere)

The reference atmosphere that aircraft performance and instruments are based on: 15 °C and 29.92 inches of mercury at sea level, cooling about 2 °C per 1,000 feet. Source: FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 4, Standard Atmosphere

Koch chart

An FAA chart that estimates how much to add to takeoff distance and subtract from climb rate for a given temperature and airport pressure altitude, for use when AFM or POH data are not available. Source: FAA pamphlet FAA-P-8740-2, Koch Chart

METAR (Aviation Routine Weather Report)

The standard coded report of the surface weather actually observed at an airport. Source: FAA Aviation Weather Handbook, 24.4

Pressure altitude

Height above the standard datum plane, where the atmosphere weighs 29.92 inches of mercury. It is what the altimeter reads when set to 29.92. Source: FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 4, Pressure Altitude

TAS (True airspeed)

The speed of the aircraft through the air mass it is flying in. In thin air it is higher than indicated airspeed. Source: FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 11, Performance Speeds

This page explains published FAA, NTSB, and NWS guidance. The calculator and every number here are estimates of the air, not of your airplane. The performance charts in your AFM or POH govern takeoff, climb, and landing. Always get a current briefing before you fly.

References

1. FAA‑H‑8083‑25C, "Pilot's Handbook of Aeronautical Knowledge." Federal Aviation Administration. 2023. Pages cited: 4-3 to 4-5 (Density Altitude; Effect of Humidity), 8-7, 10-6, 11-2 to 11-5, 11-8, 11-16, 11-17. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak. PDF: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf
2. FAA‑H‑8083‑15B, "Instrument Flying Handbook." Federal Aviation Administration. Chapter 4, pages 4-5 and 4-6 (Density Altitude; Figure 4-7, Koch chart sample). https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/FAA-H-8083-15B.pdf
3. FAA‑P‑8740‑2, "Density Altitude." Federal Aviation Administration, AFS‑8, FAA Safety Team. 2008. Pages 3 to 6. https://www.faasafety.gov/files/events/WP/WP09/2023/WP09123760/FAA-P-8740-02-DensityAltitude.pdf
4. FAA‑H‑8083‑28B, "Aviation Weather Handbook." Federal Aviation Administration, Flight Standards Service. April 2, 2026. Paragraphs cited: 3.4.2.24 (Table 3-27), 8.4.1.5; Appendix C. https://www.faa.gov/regulationspolicies/handbooksmanuals/aviation/faa-h-8083-28b-aviation-weather-handbook. PDF: https://www.faa.gov/sites/faa.gov/files/FAA-H-8083-28B.pdf
5. National Weather Service, Weather Forecast Office El Paso, "Density Altitude" calculator (Tim Brice and Todd Hall) and formula notes for station pressure, vapor pressure, virtual temperature, and density altitude. Accessed September 30, 2026. https://www.weather.gov/epz/wxcalc_densityaltitude. Formulas: https://www.weather.gov/media/epz/wxcalc/densityAltitude.pdf · https://www.weather.gov/media/epz/wxcalc/stationPressure.pdf
6. NTSB Safety Alert SA-039, "Mastering Mountain Flying." National Transportation Safety Board. March 2015. https://www.ntsb.gov/Advocacy/safety-alerts/Documents/SA-039.pdf
7. NTSB Safety Alert SA-072, "Minding Weight, Maintaining Balance." National Transportation Safety Board. February 2018. https://www.ntsb.gov/Advocacy/safety-alerts/Documents/SA-072.pdf
8. 14 CFR 91.103, "Preflight action." Electronic Code of Federal Regulations. Accessed September 30, 2026. https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F/part-91/subpart-B/section-91.103
9. FAA‑H‑8083‑2A, "Risk Management Handbook." Federal Aviation Administration. 2022. Chapter 3, "Using a Flight Risk Assessment Tool (FRAT)." https://www.faa.gov/regulationspolicies/handbooksmanuals/risk-management-handbook-faa-h-8083-2a
10. PlaneWX Help Center: "Density Altitude Caution," "FRAT," "The Risk Loop." PlaneWX. Accessed September 30, 2026. Density Altitude Caution · FRAT · The Risk Loop