Mach 9 is nine times the speed of sound, which is extremely fast and depends on the local speed of sound. In dry air at 0°C, the speed of sound is about 331.3 meters per second, so Mach 9 corresponds to roughly 2,982 meters per second, or about 10,734 kilometers per hour (about 6,675 miles per hour). In warmer air, the speed of sound is higher, so Mach 9 would be correspondingly faster in km/h, while at lower temperatures it would be slightly slower. If you need a precise value for a specific temperature and medium (for example, humidity or a different gas), the calculation uses the local speed of sound, which is defined as c ≈ sqrt(γR T) for an ideal gas, where γ is the heat capacity ratio, R is the specific gas constant, and T is absolute temperature. Then Mach 9 speed is 9c.
