Speed Converter
Convert between kilometres per hour, miles per hour, metres per second, knots and mach.
How to use this calculator
- 1Enter the value you want to convert.
- 2Pick the unit you are converting from and the one you want.
- 3The table below shows the same value in every other supported unit.
How the calculation works
result = (value × factor(from) ) / factor(to)- factor
- How many metres per second one unit represents
Every value is converted to metres per second first, then out to the target unit. This means n units need n factors rather than n² conversion pairs.
The metre per second is the SI base unit for speed; everything else scales from it.
Mach number depends on air temperature and therefore altitude. The factor here is for dry air at sea level and 15 °C; at cruising altitude the speed of sound is around 295 m/s.
Worked example
100 kilometres per hour to miles per hour
- 1.One kilometres per hour is 0.2777778 metres per second, so 100 km/h is 27.777778 metres per second.
- 2.One miles per hour is 0.44704 metres per second, so divide: 27.777778 ÷ 0.44704 = 62.137119.
- 3.Put another way, 1 km/h = 0.6213712 mph.
Result: 62.137119 mph
Speed as a ratio, not a base quantity
Speed is not a fundamental unit the way length, mass and time are — it is always a ratio of the two, distance divided by time. That is why every unit of speed is really a compound: kilometres per hour, miles per hour, metres per second. Once you know a distance unit and a time unit, a speed unit is already implied.
The SI unit is metres per second, chosen because it combines the base unit of length with the base unit of time directly, without an intermediate conversion. Every other everyday speed unit in this converter is a rescaled version of that same underlying ratio.
Why different fields settled on different units
Road speed, sea and air navigation, and physics each converged on the unit that made their own arithmetic easiest, rather than a single shared standard.
- km/h and mph — road speed limits and speedometers, split along the same metric/imperial line as most other everyday units — km/h nearly everywhere, mph in the US, UK and a small number of other countries.
- Metres and feet per second — used in physics, engineering and short-duration measurements like sprinting or falling, where a "per hour" unit would produce awkwardly small numbers.
- Knots — the standard for maritime and aviation navigation, because a nautical mile is defined from a minute of latitude, which keeps a vessel or aircraft's speed directly tied to position on a chart.
- Mach number — used in aviation and aerodynamics for high speeds, expressing velocity as a ratio to the local speed of sound rather than a fixed distance-per-time figure — a plane travelling at "Mach 1" is moving at the speed of sound wherever it happens to be flying, which changes with altitude and temperature.
Why knots and mach behave differently from the rest
Every unit in this converter except knots and mach is a fixed multiple of a metre per second, so converting between them is simple multiplication. Knots fit that same pattern — one knot is defined as exactly one nautical mile (1,852 m) per hour — but mach does not, because the speed of sound it is measured against is not a constant. Sound travels slower through cold, thin air than through warm, dense air, so the same aircraft speed corresponds to a different Mach number at altitude than it does at sea level.
This converter uses the sea-level speed of sound at 15 °C (about 340.3 m/s) as its reference, which matches how mach numbers are usually quoted for everyday reference, but treat any mach conversion as an approximation rather than an exact figure.
Common conversion pitfalls
A short list of mix-ups covers most speed-conversion confusion.
- Knots are not miles per hour — a knot (1.15 mph) is noticeably faster than a mile per hour — treating them as interchangeable understates a vessel or aircraft's actual speed.
- Mach is relative, not absolute — the same Mach number represents a different actual speed at different altitudes and temperatures, since it is scaled against the local speed of sound rather than a fixed unit.
- Wind and current speeds often use knots even in "metric" countries — meteorology and marine forecasts frequently report wind speed in knots regardless of whether the country otherwise uses km/h, because the aviation and maritime industries are knot-based worldwide.
What this assumes, and where it stops
Assumptions
- Conversion factors follow the internationally agreed definitions of each unit.
Limitations
- Mach is not a fixed unit — it varies with air temperature. Treat conversions involving it as approximate.
Common questions
How do I convert km/h to mph quickly?
Multiply by 0.62, or take about five-eighths. 100 km/h is roughly 62 mph. In the other direction, multiply mph by 1.6.
Why do ships and aircraft use knots?
One knot is one nautical mile per hour, and a nautical mile is one minute of latitude. That makes navigation arithmetic on a chart straightforward: travelling at 60 knots covers one degree of latitude in an hour.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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