Everyday Life · Unit Conversions
Unit Converter (Speed)
Convert speed values between meters per second, kilometers per hour, miles per hour, knots, and feet per second instantly.
Calculator
Formula
v_source is the input speed value, C_source is the conversion factor from the source unit to m/s, and C_target is the conversion factor from the target unit to m/s. Dividing by the target factor converts the intermediate m/s value to the desired output unit.
Source: NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), 2008 Edition.
How it works
All speed conversions in this calculator are performed by first reducing the input value to the SI base unit — meters per second (m/s) — and then multiplying by the appropriate factor to reach the target unit. This two-step approach ensures that all five units can be converted to and from one another using a single, consistent method without needing a separate conversion formula for every possible pair. The intermediate m/s value is also displayed so users can cross-reference with physics formulas that require SI units.
The conversion factors are derived from internationally agreed-upon definitions. One kilometer per hour equals exactly 1/3.6 m/s, making the km/h ↔ m/s conversion precise and exact. The international mile, defined as exactly 1609.344 metres, means 1 mph equals approximately 0.44704 m/s. The nautical mile — used in aviation and maritime navigation — is defined as exactly 1852 metres, so 1 knot equals exactly 1852/3600 ≈ 0.514444 m/s. The international foot (0.3048 m) defines 1 ft/s as exactly 0.3048 m/s. All of these definitions are standardized by NIST and the International Bureau of Weights and Measures (BIPM).
In everyday life, different industries and regions use different speed units by convention. Road speeds in the United States and United Kingdom are measured in mph, while most of the world uses km/h. Aviation and maritime navigation use knots universally, a choice that simplifies navigation calculations because one knot corresponds to one nautical mile per hour, and one nautical mile corresponds to one arcminute of latitude. Physics and engineering work in SI units, requiring m/s. This calculator bridges all those worlds in a single, streamlined tool.
Worked example
Suppose a Formula 1 car achieves a top speed of 360 km/h and you want to know the equivalent speed in miles per hour, meters per second, knots, and feet per second.
Step 1 — Convert to m/s (SI base):
360 km/h ÷ 3.6 = 100 m/s
Step 2 — Convert m/s to mph:
100 × 2.23694 = 223.694 mph
Step 3 — Convert m/s to knots:
100 × 1.94384 = 194.384 kn
Step 4 — Convert m/s to ft/s:
100 × 3.28084 = 328.084 ft/s
This means a car travelling at 360 km/h is covering roughly 100 metres every single second — equivalent to about the length of a standard football pitch per second. At 223.7 mph, this is a speed that would be illegal on any public road in the world but is commonplace during qualifying laps at circuits like Monza or Spa.
Limitations & notes
This calculator assumes constant, instantaneous speed and does not account for acceleration, deceleration, or average speed over a journey. The conversion factors used are accurate to at least 6 significant figures, which is more than sufficient for all everyday and most engineering applications; however, for extremely high-precision metrological work, users should consult the most current NIST or BIPM publications directly. The calculator does not handle relativistic speeds (a meaningful consideration only above roughly 10% of the speed of light, or ~30,000 km/s), and it does not convert angular speed (rpm, rad/s) or velocity vectors — only scalar speed magnitudes. Select inputs must be chosen before results are meaningful; leaving either unit selector on its default placeholder will yield incorrect results.
Frequently asked questions
What is the exact conversion factor between mph and km/h?
One mile per hour equals exactly 1.609344 kilometres per hour, derived from the international mile definition of exactly 1609.344 metres. Conversely, 1 km/h equals approximately 0.621371 mph. This exact relationship means the conversion is completely lossless in theory, with any rounding error introduced only by the number of decimal places you choose to display.
Why do pilots and sailors use knots instead of km/h or mph?
A knot is defined as one nautical mile per hour, and a nautical mile corresponds to one arcminute (1/60 of a degree) of latitude on the Earth's surface. This elegant relationship means that a navigator can directly relate speed to angular position on a chart without any conversion factor, simplifying dead-reckoning navigation enormously. The convention has persisted in international aviation and maritime use because it is embedded in all standard charts, instruments, and air traffic control procedures worldwide.
How fast is the speed of sound in different speed units?
The speed of sound in dry air at 20 °C (68 °F) is approximately 343 m/s, which equals about 1235 km/h, 767 mph, 667 knots, or 1125 ft/s. This value varies with temperature, humidity, and medium — in water, sound travels at roughly 1480 m/s, and in steel at around 5960 m/s. Aircraft speeds are often expressed as a Mach number, which is the ratio of the aircraft's speed to the local speed of sound at its altitude.
What is the difference between speed and velocity?
Speed is a scalar quantity — it has magnitude only and describes how fast an object is moving regardless of direction. Velocity is a vector quantity — it has both magnitude and direction, such as '60 km/h due north.' This calculator converts scalar speed values. In physics equations, when direction matters (for example, in projectile motion or orbital mechanics), velocity vectors must be handled separately, and unit conversion applies to the magnitude component of the vector.
How do I convert wind speed from knots to km/h for weather reports?
To convert knots to km/h, multiply the knot value by 1.852, since one nautical mile equals exactly 1852 metres. For example, a wind speed of 30 knots equals 30 × 1.852 = 55.56 km/h. Many weather services — including the World Meteorological Organization — report wind speeds in knots for aviation and maritime forecasts and in km/h or m/s for general public broadcasts, so this is one of the most frequently needed speed conversions in practice.
Last updated: 2025-01-15 · Formula verified against primary sources.