Supersonic speed refers to any velocity that exceeds the local speed of sound, known as Mach 1. At sea level, in standard atmospheric conditions (15°C), the speed of sound is approximately 761 mph (1,225 km/h). However, because sound travels through air, its speed changes based on temperature and altitude. As a plane climbs into the thinner, colder air of the stratosphere, the threshold for supersonic flight drops; at typical cruising altitudes of 35,000 feet, supersonic speed is roughly 660 mph (1,062 km/h). Anything traveling faster than this creates a "sonic boom" as pressure waves merge into a single shockwave. In 2026, the aviation industry is seeing a resurgence in supersonic interest with companies like Boom Supersonic testing aircraft designed to cruise at Mach 1.7, which is about 1,300 mph. It is important to distinguish this from "hypersonic" speed, which is defined as Mach 5 and above (roughly 3,800+ mph). Therefore, "how many mph" is supersonic depends entirely on the air temperature and the height at which the object is currently traveling.