Why Do Planes Cruise at 40,000 Feet? The Science of High-Altitude Flight: A Complete Guide Have you ever looked out of an airplane window and marveled at the fluffy clouds drifting below, wondering just how high you really are? For most commercial flights, that magic altitude hovers around 35,000 to 40,000 feet. While we often take this as a given when flying, the choice to soar at these impressive heights is a captivating blend of physics, economics, and engineering. It’s th