What this calculator does
Speed combines a length unit and a time unit, so every conversion here is really two conversions at once: which is why km/h to mph is not a single clean factor in the way metres to feet is, but the product of a length ratio and a time ratio.
The units in everyday use split by context: km/h and mph for road speeds, m/s for physics and engineering, knots for sea and air travel, and Mach for speeds close to or beyond the speed of sound.
The formula
Metres per second is the base unit here. Kilometres per hour converts to it by multiplying by 1,000 (metres per kilometre) and dividing by 3,600 (seconds per hour), giving the often-quoted rule of thumb that dividing km/h by 3.6 gives m/s. Knots are nautical miles per hour, and Mach is expressed relative to the speed of sound in 20°C air, taken here as a fixed 343 m/s: Mach number actually varies with altitude and temperature in reality, since the speed of sound itself depends on air density.
| Term | Meaning |
|---|---|
| Base unit | Metres per second, which every other speed unit converts through. |
| Knot | One nautical mile per hour, equal to about 1.852 km/h: used at sea and in aviation. |
| Mach | A ratio to the speed of sound, taken as 343 m/s in 20°C air for this calculator’s fixed conversion. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The value to convert. |
| From | The unit you are converting from. |
| To | The unit you want the answer in. |
When to use it
Converting a speed limit or vehicle spec between km/h and mph
Road signage and car specifications differ by country: this handles the conversion at full precision rather than the rough "divide by 1.6" shortcut.
Working with wind or boat speed in knots
Marine and aviation weather reports use knots almost exclusively; converting to km/h or mph is usually necessary to compare against a land-based forecast or a car’s speed.
Physics and engineering calculations in metres per second
m/s is the standard SI unit for speed, needed whenever a speed feeds into another physics formula like kinetic energy or momentum rather than being reported on its own.
Checking how close a speed is to the sound barrier
The Mach conversion gives a rough sense of scale for aircraft or projectile speeds: Mach 1 is roughly 1,235 km/h in 20°C air, though the true figure shifts with altitude and temperature.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
One of each unit, converted to km/h
A single unit of speed, each converted to kilometres per hour to compare true scale.
| From | In km/h |
|---|---|
| Metre per second | 3.59999712 km/h |
| Mile per hour | 1.60934271 km/h |
| Foot per second | 1.09727912 km/h |
| Knot | 1.85199692 km/h |
| Mach (20°C air) | 1,234.799012 km/h |
Converting 100 km/h to other units
A typical open-road speed limit expressed across the other common speed units.
| To | Result |
|---|---|
| Metres per second | 27.7778 m/s |
| Miles per hour | 62.13716893 mph |
| Knots | 53.99577019 kn |
Questions
How do I convert km/h to m/s quickly?
Divide by 3.6. It comes from converting 1,000 metres per kilometre and dividing by 3,600 seconds per hour, and is accurate enough for most everyday use even as a rounded rule of thumb.
What is the difference between a knot and a mile per hour?
A knot is a nautical mile per hour (1.852 km/h); a mile per hour uses the shorter statute mile (1.609 km/h). A knot is about 15 per cent faster than the same numerical value in mph.
How fast is Mach 1?
About 1,235 km/h (767 mph) in 20°C air, based on a speed of sound of 343 m/s. The true speed of sound, and so the true value of Mach 1, falls with altitude and temperature, so this is a fixed reference rather than a universal constant.
Why isn’t there a single clean factor between km/h and mph?
Because both units combine an independently defined length (kilometre or mile) with the same hour, and the kilometre-to-mile ratio itself (1.609344) is not a round number: it inherits the same non-round relationship as the underlying length units.
What speed unit should I use for physics calculations?
Metres per second: it is the SI base unit for speed and plugs directly into other SI formulas like kinetic energy or momentum without an extra conversion step.
For the distance behind any speed figure, see the length and distance converter. For a full journey calculation, use the speed, distance and time calculator.