Commercial planes typically cruise at altitudes between 36,000 and 40,000 feet. Most airliners are not allowed to fly higher than 45,000 feet. The highest altitude recorded for a commercial flight was set by the Concorde, which ascended to 60,000 feet.
While some long flights go above the average altitude, the Federal Aviation Administration allows flights that are 500 miles or shorter to fly at significantly lower altitudes of between 8,000 and 23,000 feet. This regulation was designed to cut down on heavy air traffic and lead to more on-time arrivals. However, a plane's fuel efficiency is significantly less at lower altitudes. The air is more dense, so greater energy is needed to push through it.