What is kilometre per hour?

Kilometre per hour is a unit of speed equal to one kilometre travelled in one hour. It is the most widely used unit for road speed limits, vehicle speedometers, and wind speeds in metric countries.

Real-world uses

Kilometres per hour is the standard speed unit for road traffic in most countries, displayed on car speedometers, speed limit signs, and GPS devices. Weather services report wind speeds in km/h, and athletes track running pace in min/km.

History

Kilometres per hour became widespread as the metric system was adopted for road transport in the 20th century. The first metric speed limits appeared in continental Europe. Today, km/h is used in about 91% of the world's countries for traffic regulation.

Common mistakes

Confusing km/h with m/s—100 km/h is about 27.78 m/s, not 100 m/s. Also, when converting to mph, people often use a rough factor of 1.5 instead of the correct 1.609, leading to inaccurate speed comparisons.

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.

When is this conversion used?

Speed limits, vehicle speedometers, and weather reports for wind speeds use different units depending on the country, making km/h to mph conversion one of the most practical speed conversions.

Worked examples

1 kilometre per hour = 0.00027778 kilometre per second

1 kilometre per second = 1,000 metre per second

How to convert kilometre per hour to kilometre per second

To convert kilometre per hour to kilometre per second, multiply the value by 0.00027778.

To convert kilometre per second back to kilometre per hour, multiply by 3,599.99999971.

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 hour to kilometre per second

Common values

Available Speed units

• metre per second conversions
• kilometre per hour conversions
• mile per hour conversions
• knot conversions
• foot per second conversions
• metre per minute conversions
• kilometre per second conversions
• mach (approx at sea level) conversions
• centimetre per second conversions
• mile per minute conversions

More kilometre per hour conversions

• Convert kilometre per hour to metre per second
• Convert kilometre per hour to mile per hour
• Convert kilometre per hour to knot
• Convert kilometre per hour to foot per second
• Convert kilometre per hour to metre per minute
• Convert kilometre per hour to kilometre per second
• Convert kilometre per hour to mach (approx at sea level)
• Convert kilometre per hour to centimetre per second
• Convert kilometre per hour to mile per minute

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