What is kilometre per second?

Kilometre per second is a unit of speed equal to 1,000 metres per second. It is used in astrophysics, rocket propulsion, and the measurement of planetary and cosmic velocities.

Real-world uses

Kilometres per second is used in astronomy and astrophysics for orbital velocities, galaxy recession speeds, and asteroid approach velocities. Earth orbits the Sun at about 29.78 km/s, and escape velocity from Earth is about 11.2 km/s.

History

Kilometres per second became a standard unit in celestial mechanics and astrophysics during the 20th century. It provides convenient numbers for solar system velocities, while larger cosmic speeds are often expressed as fractions of the speed of light.

Common mistakes

Forgetting the enormous scale—1 km/s = 3,600 km/h. A common error is treating km/s figures as if they were km/h. Also, confusing orbital speed with escape velocity, which are related but different concepts.

What is mach (approx at sea level)?

Mach is a dimensionless speed unit expressing the ratio of an object’s speed to the local speed of sound. At sea level, Mach 1 is approximately 343 m/s. It is used in aviation and aerodynamics to characterise high-speed flight regimes.

Real-world uses

Mach numbers are used in aerospace engineering and aviation to describe aircraft speed relative to the speed of sound. Commercial jets cruise near Mach 0.85, military fighters reach Mach 2+, and hypersonic vehicles exceed Mach 5.

History

Named after Austrian physicist Ernst Mach, who studied supersonic projectiles in the 1880s. The Mach number was formally defined by Swiss engineer Jakob Ackeret in 1929. Chuck Yeager first broke the sound barrier (Mach 1) in 1947 in the Bell X-1.

Common mistakes

Treating Mach as a fixed speed—it varies with altitude and temperature because the speed of sound changes. Mach 1 is about 343 m/s at sea level (15°C) but about 295 m/s at 11,000 m altitude.

When is this conversion used?

Converting kilometre per second to mach (approx at sea level) is useful in the speed domain when comparing values across different measurement standards or applying formulas that require a specific unit.

Worked examples

1 kilometre per second = 2.93866996 mach (approx at sea level)

1 mach (approx at sea level) = 340.29 metre per second

How to convert kilometre per second to mach (approx at sea level)

To convert kilometre per second to mach (approx at sea level), multiply the value by 2.93866996.

To convert mach (approx at sea level) back to kilometre per second, multiply by 0.34029.

Measurement standards

The SI unit of speed is metres per second (m/s), a derived unit requiring no independent definition. The knot is defined as exactly one nautical mile (1,852 metres) per hour and is sanctioned for maritime and aeronautical navigation by the International Civil Aviation Organization.

Did you know?

The fastest human-made object is the Parker Solar Probe, which reached 635,266 km/h (about 176 km/s) in 2024 during a close pass of the Sun — fast enough to travel from New York to Tokyo in under a minute.

Quick reference: kilometre per second to mach (approx at sea level)

kilometre per second	mach (approx at sea level)
0.1	0.293867
0.5	1.46933498
1	2.93866996
2	5.87733992
5	14.69334979
10	29.38669958
25	73.46674895
50	146.9334979
100	293.8669958
250	734.66748949
500	1,469.33497899
1,000	2,938.66995798

Common values

	kilometre per second	mach (approx at sea level)
Walking speed	0.0014 kilometre per second	0.00411414 mach (approx at sea level)
City speed limit	0.0139 kilometre per second	0.04084751 mach (approx at sea level)
Highway driving	0.0313 kilometre per second	0.09198037 mach (approx at sea level)
Commercial aircraft	0.25 kilometre per second	0.73466749 mach (approx at sea level)
Speed of sound (sea level)	0.34029 kilometre per second	1 mach (approx at sea level)

Available Speed units

More kilometre per second conversions

Assumption: Mach uses an approximate sea-level reference (about 340.29 m/s).