What this calculator does
Speed gets quoted in at least five units in ordinary use: metres per second in physics, kilometres per hour on road signs in most of the world, miles per hour in the US and UK, knots at sea and in the air, and feet per second in some engineering contexts. Each conversion is a fixed factor that never changes.
This converter takes one value in whichever unit you have and shows all five at once, so a figure taken from one source can be compared directly against a figure quoted in another without a second step.
The formula
Every input is first converted to kilometres per hour using its exact factor, then converted out to each of the other units. The factors are definitional: 1 m/s is exactly 3.6 km/h, 1 mile is exactly 1.609344 km, 1 knot is exactly 1.852 km/h, and 1 foot is exactly 0.3048 metres.
| Term | Meaning |
|---|---|
| Knot | One nautical mile per hour, defined as exactly 1.852 kilometres per hour. Used for ships and aircraft. |
| Metres per second | The SI unit of speed, and the one physics formulas expect. |
| Feet per second | Used in some engineering and ballistics contexts, particularly in the US. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The speed to convert. |
| From unit | The unit the value above is already in. Every other unit is calculated from it. |
When to use it
Reading a physics answer in everyday terms
Physics works in metres per second, which is hard to picture. Converting to kilometres per hour puts a result on a scale most people have an intuition for.
Comparing marine or aviation figures
Wind speeds and vessel speeds are usually quoted in knots, while forecasts for land are often in kilometres per hour, so comparing the two needs a conversion first.
Working with mixed sources
A specification in feet per second and another in metres per second describe the same thing on different scales, and converting both to a common unit is the only way to compare them honestly.
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.
What do common metres per second figures convert to?
A range of speeds in metres per second, shown in the two most commonly quoted road units.
| Metres per second | Kilometres per hour | Miles per hour |
|---|---|---|
| 1 m/s | 3.600 km/h | 2.237 mph |
| 5 m/s | 18.000 km/h | 11.185 mph |
| 10 m/s | 36.000 km/h | 22.369 mph |
| 20 m/s | 72.000 km/h | 44.739 mph |
| 30 m/s | 108.000 km/h | 67.108 mph |
Questions
Why is a knot not the same as a mile per hour?
A knot is one nautical mile per hour, and a nautical mile is longer than a statute mile. A nautical mile was originally defined as one minute of latitude, and is now fixed at exactly 1,852 metres, which makes a knot about 1.15 mph.
Is the 3.6 factor exact?
Yes. There are exactly 3,600 seconds in an hour and exactly 1,000 metres in a kilometre, so 1 m/s is exactly 3.6 km/h with no rounding involved.
Which unit should I use in a physics calculation?
Metres per second, unless the question specifies otherwise. Standard formulas assume SI units throughout, and mixing in kilometres per hour without converting is a common source of wrong answers by a factor of 3.6.
How fast is a metre per second in practice?
About 3.6 km/h, which is roughly a comfortable walking pace. Ten metres per second, at 36 km/h, is around the speed of a fast sprinter or a slow suburban car.
For speed from a distance and a time rather than a unit conversion, see the speed, distance and time calculator. For knots on their own, see the knots to km/h converter.