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MPH to Mach Converter
Use this free mph to Mach converter to turn any speed into a Mach number — or a Mach number back into mph, km/h, metres per second or knots. Mach depends on the speed of sound, which changes with air temperature and altitude, so pick the conditions that match your situation: Mach 1 is 761.2 mph at sea level in 15°C air, but only about 734.6 mph at 10,000 feet. The converter updates every unit live as you type.
The speed of sound falls as air gets colder, so the same mph is a higher Mach number at altitude.
| Kilometres per hour | 1,225.0 km/h |
| Metres per second | 340.3 m/s |
| Knots | 661.5 kn |
| Mach number | Mach 1.0000 |
How mph to Mach conversion works
Mach number is a ratio, not a fixed speed. It is the speed of an object divided by the speed of sound in the air around it, so Mach = speed ÷ speed of sound. At sea level in standard air at 15°C, sound travels at 340.3 metres per second, which is 761.2 mph or 1,225 km/h. Divide any speed in mph by 761.2 and you get its Mach number under those conditions: 380.6 mph is Mach 0.5, 761.2 mph is Mach 1, and 1,522.4 mph is Mach 2.
The catch is that the speed of sound is not a constant. Sound is a pressure wave, and it travels faster in warm air than in cold air, because warmer air molecules move faster and pass the wave along more quickly. The speed of sound in dry air follows the formula 331.3 × √(T ÷ 273.15) metres per second, where T is the temperature in kelvin. That gives 331.3 m/s (741.1 mph) at 0°C, 340.3 m/s (761.2 mph) at 15°C, and 343.2 m/s (767.8 mph) at 20°C. Because temperature falls with altitude — roughly 2°C per 1,000 feet — the speed of sound at 10,000 feet is only about 328.4 m/s (734.6 mph).
That is why pilots and engineers quote aircraft speeds in Mach rather than mph. A jet flying at 550 mph is at Mach 0.72 near sea level but Mach 0.75 at 10,000 feet, because the air up there is colder. What matters for the airframe is how fast it moves relative to the sound waves around it, not the number on a road speedometer. The converter above handles the fixed-speed units too: 1 mph = 1.60934 km/h = 0.44704 m/s = 0.86898 knots.
Speed of sound at common conditions
The table below shows the value the converter uses for each condition. Standard sea-level air at 15°C is the reference most textbooks and aviation sources use, which is why Mach 1 is usually quoted as 761 mph or 1,225 km/h. A hot day at 20°C raises it slightly to 767.8 mph, while a freezing day at 0°C drops it to 741.1 mph. At cruising altitude for airliners, around 35,000 feet, the air is near −54°C and the speed of sound is only about 660 mph — which is why a 550 mph airliner is already at Mach 0.83 up there.
The speed bands have names worth knowing. Below Mach 0.8 is subsonic — the world of cars, trains and propeller planes. Mach 0.8 to 1.2 is transonic, where airflow over parts of the aircraft goes supersonic while the plane itself has not, and drag rises sharply. Above Mach 1 is supersonic, and above Mach 5 is hypersonic. The fastest crewed aircraft, the SR-71 Blackbird, flew above Mach 3, and Concorde cruised at Mach 2.02.
Worked example
Convert 761.2 mph to Mach at sea level, 15°C, and check a car speed of 60 mph on the same scale.
Step 1: The speed of sound at 15°C is 761.2 mph.
Step 2: Divide the speed by the speed of sound: 761.2 ÷ 761.2 = Mach 1.00 — exactly the speed of sound.
Step 3: In other units, 761.2 mph is 761.2 × 1.60934 = 1,225.0 km/h, and 761.2 × 0.44704 = 340.3 m/s.
Step 4: For the car, 60 ÷ 761.2 = Mach 0.0788 — less than 8% of the speed of sound, which is why road speeds are never quoted in Mach.
Step 5: Change the condition to 10,000 feet and the same 60 mph becomes 60 ÷ 734.6 = Mach 0.0817 — a slightly higher Mach number for the same road speed, because sound is slower up there.
FAQ
How many mph is Mach 1?
At sea level in standard air at 15°C, Mach 1 — the speed of sound — is 761.2 mph (1,225 km/h, or 340.3 metres per second). The number changes with temperature and altitude: in colder air sound travels more slowly, so Mach 1 is fewer mph. At 0°C it is about 741.1 mph, and at 10,000 feet it is about 734.6 mph. Always say which conditions you mean when you quote a Mach number in mph.
What is Mach number?
Mach number is the speed of an object divided by the speed of sound in the air around it. Mach 0.8 means 80% of the speed of sound, Mach 1 means exactly the speed of sound, and Mach 2 means twice the speed of sound. Because the speed of sound changes with temperature, a Mach number describes a speed relative to the local air, not a fixed speed in mph.
What speed is 60 mph in Mach?
At 15°C sea level, 60 mph is Mach 0.0788 — just under 8% of the speed of sound. You get it by dividing 60 by 761.2, the speed of sound in mph at that temperature. Everyday speeds are tiny fractions of Mach 1: a car at highway speed is around Mach 0.09, and even a fast train is well under Mach 0.3.
Why does Mach 1 change with altitude?
The speed of sound depends on air temperature, and temperature falls as you climb. At 10,000 feet the air is around minus 5°C, so sound travels at about 734.6 mph instead of 761.2 mph at sea level. That is why a jet cruising at 550 mph is about Mach 0.72 at sea level but about Mach 0.75 at 10,000 feet — the same speed in mph is a higher Mach number in colder, thinner air.
Is Mach 1 the same as the speed of sound?
Yes, Mach 1 is defined as the speed of sound in the air where the object is flying. When a plane reaches Mach 1 it is travelling exactly as fast as sound waves move through the surrounding air. Faster than Mach 1 is supersonic, slower is subsonic, and the band around Mach 1, roughly Mach 0.8 to 1.2, is called transonic.