Speed Converter
Enter a value in any field and all other units update instantly.
Understanding Speed Units
Speed measures how quickly an object moves through space, calculated as distance divided by time. The SI unit of speed is meters per second (m/s), derived from the base units of length and time. In everyday life, however, kilometers per hour and miles per hour are far more common. Most countries worldwide use km/h for road speed limits and vehicle speedometers, while the US and UK use mph.
The knot, equal to one nautical mile per hour (approximately 1.852 km/h), is the standard speed unit in aviation and maritime navigation. Its connection to nautical miles makes it particularly practical for navigation, as one nautical mile corresponds to one minute of latitude on Earth's surface. Pilots, sailors, and meteorologists all report wind speed and travel speed in knots.
Feet per second is commonly used in ballistics, certain engineering applications, and some US sports contexts. Understanding conversions between these five speed units is valuable for international travelers, athletes tracking pace across different systems, pilots studying for licensing exams, and engineers working with specifications in different unit systems. This converter handles all five units simultaneously, updating every field instantly as you type.
Frequently Asked Questions
How do I convert km/h to mph?
To convert km/h to mph, multiply by 0.621371 (or divide by 1.60934). For example, 100 km/h equals approximately 62.14 mph. A quick mental shortcut: multiply km/h by 5/8 for a rough estimate.
What is a knot and why is it used in aviation?
A knot equals one nautical mile per hour (approximately 1.852 km/h or 1.151 mph). Knots are used in aviation and maritime navigation because nautical miles are directly related to Earth's geometry, making navigation calculations more intuitive.
What is the speed of sound in different units?
The speed of sound in dry air at 20 degrees Celsius is approximately 343 m/s, 1,235 km/h, 767 mph, 1,125 ft/s, or 667 knots. This is known as Mach 1, and it varies with temperature, humidity, and altitude.
Why does the US use miles per hour instead of km/h?
The US uses miles per hour because it never fully adopted the metric system. Speed limits, road signs, and vehicle speedometers in the US are set in mph. Most other countries use km/h. The UK uses mph for road speeds but uses metric units in most other contexts.