How high does a commercial flight fly?

Commercial aviation lives in the upper troposphere and the start of the stratosphere: between 9,000 and 12,500 meters (30,000-41,000 ft). There the air has less than half the density it has at sea level, so aerodynamic drag drops and engines burn considerably less fuel for the same speed. Above that band the wing would no longer have enough air to support the aircraft's weight without coming dangerously close to a stall.

The ceiling isn't arbitrary: every model has a certified maximum operating altitude. On modern commercial jets it's usually around 12,500 m (41,000 ft), a limit imposed both by the pressurized structure and by the requirement to be able to descend to a breathable altitude in case of depressurization.

An aircraft that has just taken off is loaded with fuel and too heavy to climb to its ceiling: it first climbs to an intermediate level and, as it burns kerosene and gets lighter, requests higher levels. This is known as the step climb, common on long routes.

Distance also matters. On a thirty-minute flight it doesn't pay to climb to 11,000 meters, because the fuel burned climbing isn't recovered by the cruise savings; that's why short regional flights often stay below 7,500 m (25,000 ft).

Above a certain altitude, flight levels are assigned with 300 meters (1,000 ft) of vertical separation between aircraft, thanks to precision altimetry systems. Air traffic control allocates those levels according to heading and congestion, so a flight's final altitude is always a negotiation between what the crew requests and what's available.