Aero Pressure Density Altitude Calculator

Pressure and density altitude: DA ≈ PA + 120·(OAT − ISA)

Pressure altitude (PA) is what the altimeter reads once you set the Kollsman window to 1013.25 hPa: PA = elevation + 27·(1013 − QNH). Density altitude (DA) then corrects that figure for a temperature that isn't standard: DA ≈ PA + 120·(OAT − ISA), with the ISA temperature at PA being 15 °C − 1.98·(PA/1000 ft). When DA climbs, the engine makes less power, the propeller bites less thrust and the wing makes less lift, so the take-off roll stretches out and the climb gradient suffers. Take El Alto / La Paz at 4060 m (13 320 ft): with an OAT of 25 °C the DA works out to roughly 5500 m (18 000 ft), high enough that a piston single can barely claw its way upward.

Applications

Flying out of high-altitude airports like La Paz, Cusco, Quito or Bogotá, running hot-and-high performance briefings across the Brazilian sertão in summer, deciding whether to launch a balloon or paramotor, revisiting weight and balance when the weather shifts, recalculating runway requirements off a NOTAM, and estimating power loss on turbocharged or normally-aspirated engines.

FAQ

Why does hot air degrade performance? Warm air is thinner. Engines pull in less oxygen, propellers and turbines push less mass each second, and at a given indicated airspeed the wing simply makes less lift.

Is DA the same as true altitude? No. DA is the altitude in the ISA atmosphere whose air density matches what you actually have outside, and on a hot day it can sit well above your real elevation.

How accurate is the 120 ft/°C rule? Under normal conditions this linear shortcut lands within a few hundred feet. For the numbers that actually matter, the AFM charts are the ones to trust.