Density Altitude Calculator
Airfield elevation, QNH and outside air temperature in; pressure altitude and density altitude out - the standard rule-of-thumb figures taught throughout PPL meteorology and performance theory (30ft per hPa, 120ft per °C of ISA deviation).
How to calculate density altitude by hand
The two-step rule of thumb behind the calculator above.
Step 1 - pressure altitude: field elevation + (1013.25 − QNH) × 30.
Step 2 - density altitude: pressure altitude + 120 × (actual OAT − ISA standard temperature at that pressure altitude), where ISA standard temperature = 15°C − 2°C per 1,000ft of pressure altitude.
Worked example: a field at 500ft, QNH 998 hPa, OAT 30°C on a warm day. Pressure altitude = 500 + (1013.25 − 998) × 30 ≈ 958ft. ISA standard temperature at 958ft ≈ 13°C, so the actual 30°C is 17°C above standard. Density altitude = 958 + 120 × 17 ≈ 2,987ft - nearly 2,500ft higher than the field's actual elevation, with the performance to match.
Frequently asked questions
What is density altitude?
Density altitude is pressure altitude corrected for temperature - it's the altitude at which the aircraft's engine and wing "feel" they're performing, in terms of standard (ISA) air density. On a hot day, density altitude can be thousands of feet higher than the airfield's actual elevation, significantly reducing performance.
What is pressure altitude?
Pressure altitude is the altitude you'd read if your altimeter were set to the standard pressure of 1013.25 hPa, rather than the local QNH. It's the first step toward density altitude, and what performance charts for take-off/landing distance are usually based on.
Why does high density altitude matter for student pilots?
Higher density altitude means less dense air for the engine to burn and the wing/propeller to work with - longer takeoff and landing distances, a reduced climb rate, and a higher true airspeed for the same indicated airspeed. It's a common factor in accidents at high-elevation or hot-and-high airfields.
Is this the exact ISA formula, or a rule of thumb?
This calculator uses the standard rule-of-thumb figures taught throughout PPL meteorology and performance theory (30ft of pressure altitude per hPa, 120ft of density altitude per °C of ISA deviation) rather than the full barometric formula - accurate enough for flight planning, and the same approximation used in ground school.