The speed at which you break the sound barrier depends on many conditions, including weather and altitude. It's approximately 770 mph or 1,239 kmh at sea level.

To break the sound barrier, an object must travel faster than the speed of sound, which is known as Mach 1. In standard atmospheric conditions (dry air at 20°C / 68°F at sea level), the speed of sound is approximately 767 mph (1,235 km/h). However, the speed of sound is not a fixed number; it varies significantly based on the temperature and altitude of the air. Because sound travels slower in colder, thinner air, a jet flying at a high altitude (around 35,000 feet) only needs to reach about 660 mph (1,062 km/h) to break the sound barrier. When an aircraft "pierces" this barrier, it creates a sudden change in pressure known as a sonic boom, which sounds like a massive explosion or a thunderclap on the ground. The first pilot to officially achieve this in level flight was Chuck Yeager in 1947, flying the Bell X-1. Breaking the barrier requires a specialized aerodynamic design to manage the intense "shock waves" and buffeting that occur as the aircraft transitions from subsonic to supersonic speeds, a challenge that famously plagued early 20th-century aviation engineers who feared the "barrier" was an impenetrable wall.