Density altitude is pressure altitude corrected for nonstandard temperature and humidity, and it is the altitude at which your airplane actually performs. On a hot day at a high-elevation field your density altitude can sit thousands of feet above your true altitude, which lengthens the takeoff roll, flattens the climb, and makes a marginal runway dangerous.
Part of our Aircraft Performance guide.
What is density altitude?
Density altitude is the altitude in the standard atmosphere at which the air density equals the density you are currently flying in. In plain terms, it is pressure altitude corrected for temperature (and, to a smaller degree, humidity). The wing, the engine, and the propeller do not care what the field elevation sign says; they respond only to how many air molecules are passing through them. When air is hot, high, or humid, those molecules spread out, the air thins, and the airplane behaves as though it were much higher up. That is why the same Cessna that leaps off a cool sea-level runway can struggle to climb out of a mountain strip on a summer afternoon.
The building block underneath density altitude is the standard atmosphere (ISA): 15 C and 29.92 in Hg at sea level, temperature dropping about 2 C per 1,000 ft, and pressure dropping about 1 in Hg per 1,000 ft. Density altitude simply measures how far today’s conditions have departed from that standard.
Why density altitude matters for performance
High density altitude reduces three things at once: wing lift, engine power, and propeller thrust. Because all three shrink together, the penalties stack up fast. Takeoff and landing distances increase, rate of climb decreases, and your climb gradient over an obstacle can shrink to almost nothing. A normally aspirated engine loses power because it is breathing thinner air, and the propeller loses bite for the same reason, so even full throttle delivers less than the book sea-level thrust.
There is a second, sneakier effect: true airspeed rises relative to indicated airspeed as density altitude increases, by roughly 2 percent per 1,000 ft of density altitude. Your airspeed indicator still shows the usual rotation and approach speeds, but the airplane is actually moving faster across the ground, so you lift off and touch down at a higher groundspeed and eat up more runway. This connects directly to how the pitot-static system reads airspeed and to how the engine makes power.
| What high density altitude does | Result |
|---|---|
| Less lift from the wing | Higher lift-off speed, longer ground roll |
| Less engine power | Weaker acceleration, lower climb rate |
| Less propeller thrust | Slower to accelerate and to climb |
| Higher true airspeed vs indicated | Faster groundspeed, more runway used |
Pressure altitude vs density altitude
Pressure altitude is your altitude referenced to the standard 29.92 in Hg datum; density altitude is that pressure altitude corrected for nonstandard temperature and humidity. Pressure altitude is purely a pressure measurement, while density altitude is what the airframe actually feels. You always find pressure altitude first, then adjust it for temperature to get density altitude. For the full method of finding pressure altitude (and why you set 29.92), see the sibling page on pressure altitude.
The density altitude formula
There are two forms of the density altitude formula every student should know: the pressure-altitude field method and the temperature rule of thumb. Used together they give the citable core of this page.
Step 1, pressure altitude:
Pressure altitude = field elevation + (29.92 minus current altimeter setting) x 1,000. If you set the altimeter to 29.92, the instrument reads pressure altitude directly.
Step 2, correct for temperature (the rule of thumb):
Density altitude = pressure altitude + 120 ft x (OAT minus ISA temperature), where the ISA (standard) temperature for your pressure altitude is 15 minus 2 x (pressure altitude in thousands of feet). In other words, add about 120 ft to pressure altitude for every 1 C the outside air is warmer than standard. If the air is colder than standard, the correction is negative and density altitude is below pressure altitude. High, hot, and humid all push density altitude up.
How to calculate density altitude step by step
Here is the full procedure carried through one running example: a field elevation of 5,000 ft, an altimeter setting of 29.42 in Hg, and an outside air temperature of 25 C.
- Note the field elevation and current altimeter setting, or set the altimeter to 29.92 to read pressure altitude directly. In our example the field elevation is 5,000 feet and the altimeter setting is 29.42 in Hg.
- Calculate pressure altitude by adding (29.92 minus the altimeter setting) times 1,000 to field elevation. Here that is 5,000 + (29.92 minus 29.42) x 1,000 = 5,500 feet.
- Find the standard ISA temperature for that pressure altitude using 15 minus 2 times the pressure altitude in thousands of feet. Here that is 15 minus (2 x 5.5) = 4 degrees Celsius.
- Find the temperature deviation by subtracting the ISA temperature from the actual outside air temperature. Here that is 25 minus 4 = 21 degrees Celsius above standard.
- Apply the rule of thumb by adding 120 feet for each degree Celsius above standard. Here that is 120 x 21 = 2,520 feet.
- Add the correction to pressure altitude to get density altitude. Here that is 5,500 + 2,520 = about 8,020 feet.
The takeaway: a 5,000 ft field on a warm day is really performing like an 8,000 ft field. Always run this before departing a short, high, or hot runway, and fold the result into your flight planning.
How to find density altitude on an E6B
The manual E6B, in paper or metal, has a small density altitude window on the calculator (slide-rule) side. Rotate the disk so the pressure altitude scale lines up against the air temperature scale: align your pressure altitude (5,500 ft) with the outside air temperature (25 C), and read density altitude in the density altitude window. For our example it shows about 8,000 ft, agreeing with the formula. The same setting also lets you read true airspeed, and the wind side handles crosswind and headwind components for the rest of your performance planning. The manual E6B does not do weight and balance, so use the weight and balance page and PHAK Chapter 10 for that. New to the whirly wheel? Start with the E6B gear overview and the sibling tutorial on using an E6B and CX-3.
How to find density altitude on a CX-3
The ASA CX-3 electronic flight computer does the same job with keystrokes instead of a slide rule, and it adds functions the manual E6B lacks. Open the Density Altitude (or DA/TAS) function, enter your pressure altitude or altimeter setting and field elevation, enter the temperature, and the CX-3 returns density altitude and true airspeed directly. Unlike the manual E6B, the CX-3 also computes weight and balance. Treat it as the upgrade: it is faster and less error-prone once you understand what the numbers mean, but learn the hand method first so you can sanity-check the box. Both the manual E6B and the CX-3 are permitted on the FAA knowledge and practical tests; phone and tablet apps are not permitted on the FAA knowledge test.
How to read a density altitude chart (and the Koch chart)
A density altitude conversion chart (the kind printed in the Pilot’s Handbook of Aeronautical Knowledge and many POHs) turns the two-step math into one graph. Find the outside air temperature along the bottom scale, move straight up to the diagonal line for your pressure altitude, then read across to the left to get density altitude. Entering our example (25 C, pressure altitude 5,500 ft) lands you at about 8,000 ft, matching both the formula and the E6B.
The Koch chart answers a different question: how much does that density altitude cost you on takeoff and climb? Draw a line from the temperature scale on the left to the pressure altitude scale on the right, and where it crosses the two center scales you read the percent increase in takeoff distance and the percent decrease in rate of climb. At a warm, several-thousand-foot density altitude the Koch chart typically shows takeoff distance roughly doubling and climb rate falling by something like one third to one half. Those are approximations for awareness only; for legal planning use your airplane’s actual charts, covered on the takeoff and landing performance page.
Worked examples: formula, E6B, and chart agree
Running the same conditions (field elevation 5,000 ft, altimeter 29.42, OAT 25 C, pressure altitude 5,500 ft) through all three methods gives essentially the same answer, which is exactly what you want to see on a checkride.
| Method | Density altitude |
|---|---|
| Formula (rule of thumb, 120 ft per degree) | about 8,020 ft |
| Manual E6B (align PA 5,500 with OAT 25 C) | about 8,000 ft |
| Density altitude conversion chart | about 8,000 ft |
Small differences of a hundred feet or so are normal between methods and rounding; the point is that all three land in the same neighborhood, and all three say this 5,000 ft field is really an 8,000 ft field today.
High density altitude effects and a hot-and-high pre-takeoff checklist
When density altitude is high, plan for the airplane to accelerate slowly, use far more runway, and climb weakly, and remember that your groundspeed at lift-off is higher than the airspeed indicator suggests. Thin air also affects mixture: at a high density altitude a full-rich mixture is too rich, which robs power and can foul plugs. Use this checklist before a hot-and-high departure:
- Calculate density altitude first and then pull actual takeoff and climb numbers from the POH for that density altitude, weight, and wind.
- Lean the mixture for best power on the ground before takeoff when the field density altitude is high (follow your POH procedure), so the engine makes the power it can.
- Reduce weight if the numbers are marginal. Offload baggage, carry less fuel to your first stop, or take fewer passengers. Weight is the one variable fully under your control, so pair this with your weight and balance workup.
- Use all available runway and the full recommended technique, including the soft-field or short-field procedure when appropriate. See short and soft field takeoffs and landings.
- Favor the cooler hours. Departing early morning instead of mid-afternoon can drop density altitude by thousands of feet.
- Add a personal margin to book distances (commonly 50 percent or more), because POH numbers assume a new airplane and a test pilot.
- When in doubt, wait or offload. If the runway, obstacles, and climb gradient do not add up with margin, do not go. Density altitude is a recognized contributor to takeoff and climb accidents, a theme that ties into other weather hazards.
Density altitude on the checkride
Expect density altitude to appear on both the oral and the flight. Examiners want to hear the definition in one clean sentence, watch you compute it from an altimeter setting and temperature, and, most importantly, hear you connect the number to a go or no-go decision for the day’s runway and airplane. Be ready to explain why high density altitude reduces lift, power, and thrust, why true airspeed climbs relative to indicated, and how leaning and weight reduction help. Review the standards on the Private Pilot ACS page as you prepare.
What you'll need
These are the tools student pilots use to run density altitude and the rest of their performance numbers, from PilotMall.com.
Frequently asked questions
What is considered a high density altitude?
Any density altitude above about 5,000 feet is generally treated as high, and on a hot day the effects on a light airplane become serious well before that. There is no single legal threshold, so the practical rule is that whenever density altitude climbs a few thousand feet above your field elevation, expect noticeably longer takeoff rolls and weaker climb performance.
How do you calculate density altitude quickly in your head?
Add about 120 feet to your pressure altitude for every 1 degree Celsius the outside air is warmer than standard for that altitude. Find the standard temperature as 15 minus 2 times the pressure altitude in thousands of feet, subtract it from the actual outside air temperature, multiply the difference by 120, and add that to pressure altitude.
What is the difference between pressure altitude and density altitude?
Pressure altitude is your altitude referenced to the standard 29.92 in Hg datum, while density altitude is that pressure altitude corrected for nonstandard temperature and humidity. Pressure altitude is a pressure measurement, and density altitude is what the airplane actually feels, so on a hot day density altitude can be thousands of feet higher than pressure altitude.
Does humidity increase density altitude?
Yes, high humidity increases density altitude because water vapor is less dense than dry air, so moist air behaves like thinner, higher air. The manual E6B and the standard charts ignore humidity, so on a hot, humid day add extra margin because your true density altitude is a little higher than the chart shows.
How does high density altitude affect takeoff and climb?
High density altitude reduces wing lift, engine power, and propeller thrust, so takeoff and landing distances increase and rate of climb decreases. Your true airspeed is also higher than indicated (about 2 percent per 1,000 feet of density altitude), so you lift off at a faster groundspeed and need more runway.