Speed of Sound in mph

How fast is the speed of sound in mph?

The speed of sound is 761 mph (1,225 km/h) at sea level on a standard 15 °C (59 °F) day. NASA’s Glenn Research Center gives the same figure, 761 mph, for “standard day, sea level static conditions”. In metric that is 340.3 meters per second, 661 knots, or about 1,116 feet per second.

The converter above starts at Mach 1 and shows the sea-level value in mph. Type any Mach number to see it in mph, or tap Swap to turn an mph speed into a Mach number. The converter uses the ICAO and US Standard Atmosphere sea-level value of 340.294 m/s for Mach 1, the same constant our speed converter uses, and rounds only for display.

The catch: Mach 1 is not one speed. It is the local speed of sound, and that changes with the temperature of the air. A jet flying at Mach 0.85 at 39,000 ft is going about 561 mph, not the 647 mph you would get with the sea-level number.

Why does the speed of sound change with temperature?

Sound is a pressure wave carried by colliding air molecules, and warmer molecules move faster, so the wave travels faster. NASA writes the relationship as one formula:

a = √(γ × R × T)

• a is the speed of sound in m/s.
• γ (gamma) is the ratio of specific heats, 1.4 for air.
• R is the gas constant for air, 287.05 J/(kg·K).
• T is the absolute temperature in kelvin (°C + 273.15).

Check it at sea level: √(1.4 × 287.05287 × 288.15) = 340.294 m/s, exactly the standard value. Pressure and density do not appear: they cancel out for an ideal gas, so thin air at the same temperature carries sound just as fast as dense air.

Because speed goes with the square root of temperature, the effect is modest but real. Near room temperature, each 1 °C adds about 0.59 m/s (1.3 mph):

Computed with a = √(γRT), γ = 1.4, R = 287.05287 J/(kg·K), dry air. Humidity raises the value slightly (a fraction of a percent), which this table ignores.

What is the speed of sound at altitude?

In the standard atmosphere the speed of sound falls from 761 mph at sea level to 660 mph (1,062 km/h) at 36,089 ft, then stays at 660 mph up to 65,617 ft. The reason is the temperature profile: the 1976 US Standard Atmosphere cools air by 6.5 °C per kilometer up to 11 km (36,089 ft), holds it at −56.5 °C to 20 km, then warms it slowly.

Altitudes are geopotential; values are for the standard day. Real air on a given day can be 10 to 20 °C warmer or colder, which shifts Mach 1 by roughly 13 to 26 mph. This is why pilots read Mach from instruments that measure the air around them, and why the ground speed your phone shows on a plane is a different number again (see how fast do planes fly).

How do you convert Mach to mph?

Multiply the Mach number by the local speed of sound: at sea level, mph = Mach × 761.2; above 36,089 ft, mph = Mach × 660.1. The converter above uses the sea-level constant; use the altitude column for cruise conditions.

NASA defines the regimes by Mach number: subsonic below 1, transonic near 1, supersonic above 1, and hypersonic “typically greater than Mach 5.0”. Note that the X-59 row agrees with NASA’s own figure: Mach 1.4 at 55,000 ft is 924 to 925 mph, because at that height Mach 1 is 660 mph. To turn any of these into knots for aviation use, see the knots to mph converter.

What happens when a plane breaks the sound barrier?

Above Mach 1 an aircraft outruns its own pressure waves; they merge into shock waves that trail behind it in a cone, and when that cone sweeps the ground people hear a sonic boom. The boom is not a one-off event at the moment of “breaking” the barrier: it follows the aircraft along its whole supersonic track.

That boom is why supersonic travel over land is still restricted. Under 14 CFR 91.817, civil aircraft may not fly faster than Mach 1 over the United States without an FAA authorization, and NASA notes the FAA “currently prohibits supersonic flight over land due to the loud boom”.

NASA’s answer is the X-59 of the Quesst mission, a 99.7-ft, single-pilot jet shaped to turn the boom into a quiet “thump”. NASA’s design target is cruise at Mach 1.4 (925 mph) at 55,000 ft. The aircraft first went supersonic on 5 June 2026, according to AIAA’s report of NASA’s announcement; flights over communities to measure how people perceive the thump are meant to feed new noise standards.

How far does sound travel in a second?

At 15 °C sound covers 340 m (1,116 ft) per second, so it takes about 4.7 seconds to travel one mile and 2.9 seconds to travel one kilometer. That gives the classic thunderstorm rule: count the seconds between lightning and thunder, divide by 5 for miles or by 3 for kilometers.

Some other useful numbers:

• Echo distance: a clap that returns in 1 second came off a wall about 170 m (558 ft) away, half the round trip.
• Stadium delay: from the far end of a 100 m field, a starter’s pistol arrives about 0.29 s late, a big gap in a race timed to 0.01 s.
• Sound vs. speed: to convert any measured speed of sound in m/s into mph, use the m/s and mph converter.

How to use the Mach to mph converter

Enter a Mach number in the first box and read mph next to it; the sea-level standard (Mach 1 = 761.2 mph) is used.

1. Type a Mach number, for example 0.85 or 2.
2. Read mph, or tap All units for km/h, knots, m/s and ft/s at once.
3. Adjust for altitude with the tables above: multiply by 660.1 instead of 761.2 for anything flying above 36,089 ft.
4. Swap to turn an mph speed into a Mach number, such as a car at 300 mph (Mach 0.39 at sea level).