Enter field elevation, altimeter setting, and temperature. Get pressure altitude and density altitude instantly — and see how much runway performance you're really working with.
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Pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000. Density altitude ≈ pressure altitude + 120 × (OAT − ISA temperature). A 5,000-ft airport at 30 °C with the altimeter at 29.92 works out to a density altitude around 8,000 ft — your airplane takes off and climbs as if the airport moved 3,000 feet higher. Run your numbers below, then check the POH at the density altitude, not the field elevation.
From the chart supplement or your EFB
From the METAR / AWOS ("A2992" → 29.92)
Outside air temperature at the field
Used only to flag high humidity
Density altitude is pressure altitude corrected for non-standard temperature — the altitude in the standard atmosphere at which the air would be as thin as it is right now at your airport. Your wing, propeller, and normally aspirated engine do not care about the number painted on the airport sign; they care about air density. When the air is hot, the airplane performs as if the airport were much higher, even though your altimeter still reads field elevation on the ramp.
Two steps get you the number:
This is the standard rule-of-thumb formula from the FAA handbooks. A Koch chart, E6B, or your EFB may read slightly higher because they model the atmosphere more exactly — treat the rule of thumb as a floor, not a precise answer.
Say you're departing a 5,000-ft-elevation airport on a summer afternoon: altimeter 29.92, temperature 30 °C.
The airplane you flight-planned for a 5,000-ft airport is actually operating around 8,000 ft — and a chart or E6B will put it a few hundred feet higher still. If your POH takeoff table stops looking comfortable at that line, that is your answer.
Thin air hits you three ways at once. The engine develops less power because each intake stroke pulls in fewer air molecules (a normally aspirated engine loses power on the order of a few percent per 1,000 ft of density altitude). The propeller is a rotating wing, so it produces less thrust in the same thin air. And the wing needs a higher true airspeed — meaning a longer ground roll — to produce the same lift. Humidity piles on quietly: water vapor is lighter than dry air, so muggy air is less dense still and the engine loses additional power. The rule-of-thumb formula ignores humidity entirely, which is one more reason to leave margin on hot, humid days.
The insidious part is that nothing feels wrong. Indicated airspeeds for rotation and climb are the same as always; the airplane simply accelerates slower, uses more runway, and climbs at a fraction of the book sea-level rate.
Density altitude accidents follow a script the NTSB has documented for decades: a high-elevation airport on a hot afternoon, an airplane loaded to (or past) gross weight, a runway that would be fine on a cool morning, and rising terrain off the departure end. The takeoff roll is long but the airplane flies off — then it won't climb out of ground effect, or climbs at 150 fpm toward terrain that rises faster. Each factor alone is manageable; stacked together they close the escape routes. The standard outs are equally well known: depart in the cool of early morning, offload fuel or people, use the longest runway, and lean the mixture for maximum power per the POH before takeoff at high density altitude.
Many AWOS/ASOS broadcasts announce density altitude directly ("density altitude two thousand eight hundred") whenever it is well above field elevation — listen for it at the end of the broadcast. If it isn't announced, pull the two numbers you need from the METAR: the altimeter setting (the group starting with "A", so A2992 is 29.92 inHg) and the temperature (the first half of the temperature/dew-point group, so 25/12 is 25 °C). Combine those with field elevation in the calculator above and you have it in seconds.
Exact numbers belong to your POH, but these directional effects hold for typical normally aspirated GA airplanes:
| Per ~1,000 ft of density altitude | Effect |
|---|---|
| Engine power (normally aspirated) | Drops roughly 3% |
| Takeoff ground roll | Grows meaningfully — check the POH table, and add the FAA's 50% safety margin on top |
| Rate of climb | Shrinks steadily; many trainers lose most of their sea-level climb by 8,000–10,000 ft DA |
| True airspeed at rotation | Higher for the same indicated speed — the ground roll is longer even before the power loss |
The FAA's Koch chart (in the Density Altitude pamphlet) makes the compounding vivid: its classic example — a 6,000-ft airport at 100 °F — shows takeoff distance increasing by roughly 230% and rate of climb decreasing by roughly 75% versus sea-level standard. If a chart tells you your climb rate drops by three quarters, believe it before the runway does.
Common QuestionsIt is the altitude your airplane thinks it is at — pressure altitude corrected for temperature. On a hot day the air is thinner than the field elevation suggests, so the wing, propeller, and engine perform as if the airport were sitting thousands of feet higher. A 5,000-foot airport at 30 °C performs like an airport near 8,000 feet.
Take the altimeter setting and temperature from the METAR and the field elevation from the chart supplement or your EFB. Pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000. Then density altitude ≈ pressure altitude + 120 × (OAT − ISA temp), where ISA temp = 15 − 2 × (pressure altitude ÷ 1,000). Many AWOS/ASOS stations also broadcast density altitude directly when it is well above field elevation.
Yes, but less than temperature does. Water vapor is lighter than dry air, so humid air is slightly less dense — high humidity adds a few hundred feet of effective density altitude on a hot day and measurably cuts engine power. The rule-of-thumb formula (and this calculator) does not include humidity, so on hot, muggy days treat the computed number as slightly optimistic.
There is no single regulatory threshold — it depends on your aircraft, weight, and runway. As working guidance, start paying close attention whenever density altitude sits several thousand feet above field elevation or above roughly 5,000–6,000 ft at your weight, and run actual POH takeoff and climb numbers instead of estimating. Many mountain-flying instructors treat high DA + heavy airplane + short runway as a no-go until at least one of the three improves.
Three things degrade at once: the engine makes less power in thinner air, the propeller produces less thrust, and the wing needs a higher true airspeed for the same lift. The result is a longer takeoff roll and a climb rate that can shrink to a fraction of the sea-level book number — while indicated airspeeds and technique feel deceptively normal.
Headwind and crosswind components from runway, wind, and gusts — instantly.
Set your own ceiling, visibility, wind, and density-altitude numbers before the pressure to fly shows up.
A repeatable structure for the decision this calculator feeds into.
FlightDecide computes density altitude and runway performance for your specific aircraft and flight window — alongside winds, ceilings, visibility, NOTAMs, fuel, and W&B — then gives you a GO / CAUTION / NO GO advisory you can verify against the raw data.
Get FlightDecide on the App StoreEducational tool for flight-planning practice. It is advisory only and not a substitute for your POH, an official weather briefing, or your own judgment as pilot in command (14 CFR 91.3). Sources: FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Ch. 4 & 11; FAA Density Altitude pamphlet (FAA-P-8740-2). Last reviewed: July 17, 2026.