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density altitude calculator

How high does the aeroplane think it is? Live airport figures or your own numbers, with pressure altitude, ISA deviation and what it means for performance.

Pulls elevation, temperature, QNH and dew point from the latest METAR.

Adds a humidity-corrected figure alongside.

Sea-level pressure, not the station pressure.

Pressure unit

Switching converts the value you have typed.

Density altitude

8,503ft

Significantly degraded: 3,069 ft above the field

Pressure altitude

5,441 ft

Field corrected to the 1013.25 datum

ISA temperature

4.2 °C

Standard temperature at that pressure altitude

ISA deviation

+25.8 °C

Warmer than standard

Rough expectation: around 85% more take-off roll and about 60% less rate of climb than a sea-level standard day. These are rules of thumb for orientation only. The numbers that matter are in your aircraft’s performance charts.

What the three numbers mean

Pressure altitude is your field elevation re-referenced to the standard 1013.25 hPa datum, so low pressure raises it. ISA deviation is how much warmer or colder the day is than the standard atmosphere at that height. Density altitude combines the two: it is the altitude at which the standard atmosphere has the density your aircraft is actually flying in. Wings, propeller and engine all care about density, not elevation, which is why a hot day at a high field flies like a much higher one.

The arithmetic is two lines. Pressure altitude = elevation + (1013.25 − QNH) × 27.3 ft per hPa. Density altitude = pressure altitude + 118.8 × ISA deviation in °C. Both are the standard approximations, which is deliberate: they are what POH charts and examiners agree on.

Data: NOAA Aviation Weather Center, refreshed on demand. For training and reference only: always use an approved briefing source and your aircraft’s charts for operational decisions.

Density altitude is the altitude at which the standard atmosphere has the air density you are actually flying in. Wings, propeller and engine all care about density rather than elevation, which is why a hot day at a high field flies like a much higher one. Two corrections get you there: pressure, then temperature.

Key takeaways

  • Pressure altitude = elevation + (1013.25 − QNH) × 27.3 feet per hectopascal.
  • Density altitude = pressure altitude + 118.8 feet per °C of ISA deviation.
  • The standard atmosphere is 15 °C at sea level, cooling 1.98 °C per thousand feet. ISA deviation is how far the real day departs from it.
  • Rough orientation: about 10% more take-off roll and 7% less rate of climb per thousand feet of density altitude.
  • Temperature does most of the work. Denver on a 30 °C day with a standard QNH sits near 8,500 feet of density altitude, three thousand feet above the field itself.

The two corrections, in order

First, pressure. Your altimeter is a barometer, and the number it shows depends on the datum you set. Setting 1013.25 gives you pressure altitude: your height above the standard pressure datum rather than above the ground. Low pressure raises it, at about 27 feet per hectopascal, which is why a deep low can put you a few hundred feet higher than the field elevation before the temperature is even considered.

Then, temperature. The standard atmosphere says the air at that pressure altitude should be a particular temperature. If it is warmer, the air is thinner than standard, and the correction is 118.8 feet for every degree of excess. This is the term that dominates: a 25 °C departure from standard is worth nearly three thousand feet on its own.

What it actually does to the aeroplane

Thin air affects three things at once, which is why the effect compounds. The propeller has less air to work on, so it produces less thrust. A normally aspirated engine draws in less mass of air, so it makes less power. And the wing needs a higher true airspeed to generate the same lift, so you accelerate to a higher groundspeed before you fly.

The consequence is a longer ground roll, a shallower climb gradient, and a much longer distance to clear an obstacle. The indicated airspeeds do not change: you rotate at the same number on the dial, but you are travelling faster over the ground, using more runway and climbing more slowly once airborne. Density altitude accidents are usually obstacle accidents rather than stall accidents.

Where humidity comes in

Water vapour is lighter than the nitrogen and oxygen it displaces, so humid air is less dense than dry air at the same temperature and pressure. The standard density altitude formula ignores it entirely. Enter a dew point and this calculator adds a humidity-corrected figure derived from the vapour pressure, which on a hot humid day sits a few hundred feet above the standard one.

Plan with the standard figure, because that is the one your performance charts were built from and the one an examiner will expect. Treat the humid figure as a reminder that the margin is thinner than the chart suggests, in the same way you would treat a soft surface or a slight upslope.

Reading the answer honestly

The banding on this page is deliberately based on how far the density altitude sits above the field, not on its absolute value. Eight thousand feet of density altitude is routine at a Colorado airfield and remarkable at a Kent one, and it is the gap between the two that tells you how much the aeroplane will be misbehaving compared with what you are used to.

The performance percentages on this page are rules of thumb for orientation, nothing more. Every airframe, propeller and engine combination differs, and the only numbers you should plan a take-off with are the ones in your own flight manual, interpolated properly, with a margin added for a soft surface, a slope, or a runway you have not used before.

Frequently asked questions

How do you calculate density altitude?

In two steps. First correct the field elevation to the standard pressure datum: pressure altitude equals elevation plus 27.3 feet for every hectopascal the QNH sits below 1013.25. Then correct for temperature: density altitude equals pressure altitude plus 118.8 feet for every degree Celsius the air is warmer than the standard atmosphere at that pressure altitude. Colder than standard and the correction goes the other way.

What is the ISA standard atmosphere?

A reference atmosphere of 15 degrees Celsius and 1013.25 hectopascals at sea level, cooling at 1.98 degrees Celsius per thousand feet. It is not a forecast of anything. It exists so that altimeters, performance charts and pilots all use the same yardstick, and so that a real day can be described as a deviation from it.

What is a high density altitude?

It depends on the field, which is why this calculator bands the figure by how far it sits above the field rather than by its absolute value. A density altitude a thousand feet above the elevation is barely noticeable; four thousand feet above it will change how the aeroplane climbs, accelerates and handles quite dramatically. A sea-level airfield on a hot day can easily produce three thousand feet of density altitude.

How does density altitude affect take-off performance?

Thin air means less thrust from the propeller, less power from the engine and less lift at the same indicated airspeed, so the aeroplane accelerates more slowly, lifts off at a higher groundspeed and climbs worse. As a rough orientation, expect roughly ten per cent more take-off roll and about seven per cent less rate of climb for every thousand feet of density altitude. Those are rules of thumb: use your aircraft's performance charts for the real figures.

Does humidity affect density altitude?

Yes, though less than temperature. Water vapour is lighter than the dry air it displaces, so moist air is less dense and the true density altitude is a few hundred feet higher than the standard calculation on a hot humid day. Enter a dew point and this calculator shows the humidity-corrected figure alongside. Plan with the standard figure, because that is what your performance charts were built on.

Is density altitude the same as pressure altitude?

Only on a standard day. Pressure altitude is your height above the 1013.25 datum and ignores temperature entirely. Density altitude is pressure altitude corrected for how far the temperature departs from standard. On a cold day density altitude sits below pressure altitude, which is why a winter departure feels so much livelier.

Why can I not use QNH as the station pressure?

Because QNH has already been corrected to sea level. At a high-elevation airfield the actual pressure at the field is far lower than the QNH suggests: at Denver the difference is roughly 180 hectopascals. Using QNH as the station pressure would understate density altitude by thousands of feet, so this calculator derives the station pressure from the pressure altitude instead.

Should I use the temperature in the METAR or the one on the ramp?

Whichever is closer to the air the aeroplane will actually fly through. A METAR temperature comes from a properly sited and shaded sensor and is the right figure for planning. A thermometer sitting on black tarmac in the sun reads high. If the two disagree by a lot, plan with the higher figure and be pleasantly surprised.

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