How to use this calculator
- Enter field elevation. Use the airport or runway elevation.
- Enter altimeter setting. Use the current pressure setting in inches of mercury.
- Enter OAT. Use outside air temperature in degrees C.
- Read density altitude. Use the highlighted density altitude and compare it with the 120 ft/C estimate as performance context only.
How it works
Pressure altitude is estimated as field elevation + (29.92 - altimeter) x 1000 ft. ISA temperature is 15 - 0.0019812 x pressure altitude_ft. The primary result computes dry-air density from ISA pressure at pressure altitude and OAT, then converts that density ratio back to the equivalent ISA altitude.
The comparison output keeps the familiar quick estimate DA ~= PA + 120 x (OAT - ISA temperature). It is close for many training examples, but the density-derived value is the better default when this is used as a calculator rather than a mental shortcut.
Use it with climb gradient, stall speed and weight and balance.
Worked example
Verified against the live calculator
At a 5,000 ft field, 29.92 inHg altimeter and 30 C OAT, ISA temperature is about 5 C and the dry-air density altitude is about 7,800 ft. The 120 ft/C quick estimate is about 8,000 ft.
Frequently asked questions
How is density altitude calculated here?
It estimates pressure altitude from field elevation and altimeter setting, computes dry-air density from ISA pressure and actual OAT, then finds the ISA altitude with the same density.
Why is there also a 120 ft/C estimate?
The 120 ft/C output is the common quick rule from pressure altitude and ISA temperature deviation. It is useful for a cockpit-style cross-check, but the highlighted value uses the dry-air density inversion.
What is pressure altitude?
Pressure altitude is field elevation corrected to a standard 29.92 inHg pressure setting.
Does humidity matter?
Humidity can affect density and aircraft performance. This calculator is a dry-air screen and does not model dew point or water vapor.
Can this replace takeoff performance charts?
No. Use approved performance charts, current weather and runway data for operational planning.
Method & assumptions
- Uses dry air, ISA pressure at the estimated pressure altitude and actual outside air temperature.
- Altimeter-setting pressure altitude uses the common 1 inHg = 1,000 ft screening shortcut.
- Does not model humidity, runway slope, wind, surface condition, aircraft weight/configuration or engine/propeller-specific performance.
- Does not replace aircraft performance charts, runway analysis, NOTAMs, weather observations, POH/AFM data or flight manual limitations.