What is that in Mach numbers?
Mach 1 is not a fixed speed — it falls with temperature, so an aircraft goes supersonic more easily at altitude.
2 × a speed of sound
686 m/s ÷ 343 m/s = 2 Machs.
Reference figures are measurements and are rounded; the unit conversion behind them is exact.
| Reference | Size | Yours ÷ theirs | Note |
|---|---|---|---|
| airliner at cruise | 250 m/s | 2.74× | |
| speed of sound at altitude | 295 m/s | 2.33× | At 11 km, where airliners cruise |
| speed of sound | 343 m/s | 2× | Dry air at 20 °C at sea level |
| Concorde | 604 m/s | 1.14× | |
| SR-71 Blackbird | 980 m/s | 0.7× |
Worth knowing
- Mach 1 is 1 235 km/h at sea level but only 1 062 km/h at cruising altitude, because sound travels more slowly in colder air.
- Concorde cruised at Mach 2.04. No civil aircraft has flown supersonically in commercial service since it retired in 2003.
Frequently asked
How fast is Mach 1?
1 235 km/h (767 mph) in dry air at 20 °C. It depends only on temperature, not on air pressure or altitude directly.
What is the sound barrier?
The sharp rise in drag as an aircraft approaches Mach 1 and pressure waves pile up into a shock. Once through it, drag falls again — which is why "breaking" it is the right word.