How to convert speed units
A speed converter changes a rate of travel from one unit to another: kilometers per hour, miles per hour, meters per second, feet per second or knots. Type the speed, pick the From and To units, and press Convert. Everything passes through meters per second, the SI unit of speed, and results show up to 8 decimal places.
Each speed unit is a length unit divided by a time unit, so the factors come from the definitions of the mile, foot and nautical mile combined with 3,600 seconds in an hour.
Exact conversion factors
| Unit | Symbol | In m/s | Where it comes from |
|---|---|---|---|
| Meter per second | m/s | 1 | SI base units |
| Kilometer per hour | km/h | 0.2777… (1/3.6) | 1,000 m ÷ 3,600 s |
| Mile per hour | mph | 0.44704 | 1,609.344 m ÷ 3,600 s |
| Foot per second | ft/s | 0.3048 | 0.3048 m ÷ 1 s |
| Knot | kn | 0.5144… | 1,852 m ÷ 3,600 s |
The mile (1,609.344 m) and foot (0.3048 m) have been exact since the 1959 international yard agreement, and the nautical mile was set at exactly 1,852 m by international agreement in 1929. NIST SP 811 lists all three alongside the SI units, which are defined in the BIPM SI Brochure. The km/h and knot factors repeat forever as decimals, which is why the converter stores them to 12 decimal places; the results still match the exact ratios to the 8 places displayed.
Quick reference
| From | To | Multiply by |
|---|---|---|
| km/h | mph | 0.621371 |
| mph | km/h | 1.609344 |
| m/s | km/h | 3.6 |
| km/h | m/s | 0.277778 |
| m/s | mph | 2.23694 |
| mph | ft/s | 1.46667 |
| knots | km/h | 1.852 |
| knots | mph | 1.15078 |
| m/s | knots | 1.94384 |
Common speeds
| Context | km/h | mph | m/s |
|---|---|---|---|
| Common urban limit (many countries) | 50 | 31.07 | 13.89 |
| Common US highway limit | 104.61 | 65 | 29.06 |
| Common motorway limit (many countries) | 120 | 74.56 | 33.33 |
| UK national motorway limit | 112.65 | 70 | 31.29 |
Speed limits are set by each country and change over time, so treat this table as a set of typical values rather than the current law for any particular road.
Worked examples
- Driving abroad: a 120 km/h sign in mph is 120 ÷ 1.609344 = 74.56 mph.
- A US speedometer reading of 70 mph in km/h: 70 × 1.609344 = 112.65408 km/h.
- A sprinter averaging 9.58 m/s: 9.58 × 3.6 = 34.488 km/h.
- A 30-knot wind in km/h: 30 × 1.852 = 55.56 km/h.
- A 5-knot current in m/s: 5 × 1,852 ÷ 3,600 = 2.5722 m/s.
Which unit is used where
Road signs use km/h almost everywhere, with mph in the United States and the United Kingdom. Physics and engineering work in m/s. Ships and aircraft use knots because a nautical mile is roughly one minute of latitude, which makes distances on charts easy to read off. Weather services vary: some report wind in km/h, others in knots, mph or m/s, so check which one a forecast uses before comparing.
Feet per second turns up in ballistics, water flow and some engineering specifications. One mph is exactly 22/15 ft/s, about 1.4667, a handy number for working out how far a car travels in a second.
Speed, distance and time
Speed only tells you the rate. To get a journey time from it, divide distance by speed, or let the travel time calculator do it with rest stops included. If your distance is in a different unit from the speed, convert it first with the length converter.
Related pages
- Travel time calculator: hours and minutes for a trip at a given average speed.
- Length converter: miles, kilometers, feet and meters.
- Fuel cost calculator: what a drive will cost in fuel.
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