Speed Converter
Rental cars, weather reports and marine charts all speak different speed units. Enter one value to see km/h, mph, knots and meters per second at once — useful when a road sign, a forecast and a boat's instruments disagree.
Last updated — conversions use exact unit definitions, so results stay accurate.
| Unit | Value |
|---|
How it works
Speed is distance divided by time, so converting it means converting both parts. The clean route is through meters per second: 1 km/h is 0.277778 m/s and 1 mph is exactly 0.44704 m/s (because a mile is 1,609.344 m and an hour is 3,600 seconds).
The everyday conversion is km/h to mph: multiply by 0.621371. A 100 km/h speed limit is about 62 mph. Going the other way, multiply mph by 1.609344. A useful mental shortcut: the Fibonacci sequence approximates it well — 5 mph ≈ 8 km/h, 8 mph ≈ 13 km/h, 13 mph ≈ 21 km/h.
Knots are nautical miles per hour, so 1 knot is 1.852 km/h or 1.15078 mph. Knots are used at sea and in aviation because they map to minutes of latitude, which makes chart navigation simpler. Wind speeds in marine forecasts are almost always given in knots, while land forecasts use km/h or mph.
Meters per second is the scientific standard and the unit physics problems expect. To convert from km/h, divide by 3.6: 100 km/h is 27.78 m/s. That factor of 3.6 is worth memorising, since it appears in everything from sprint times to stopping distances.
Speed reference points
| km/h | mph | knots | m/s | Context |
|---|---|---|---|---|
| 5 | 3.1 | 2.7 | 1.39 | Brisk walking speed |
| 20 | 12.4 | 10.8 | 5.56 | City cycling |
| 50 | 31.1 | 27.0 | 13.89 | Urban speed limit |
| 100 | 62.1 | 54.0 | 27.78 | Highway / motorway |
| 120 | 74.6 | 64.8 | 33.33 | Motorway limit (many countries) |
| 300 | 186.4 | 162.0 | 83.33 | High-speed rail |
| 900 | 559.2 | 486.0 | 250.0 | Commercial jet cruise |
| 1,225 | 761.2 | 661.5 | 340.3 | Speed of sound at sea level |
Frequently asked questions
How do I convert km/h to mph?
Multiply kilometers per hour by 0.621371. So 100 km/h × 0.621371 = 62.1 mph. To convert mph to km/h, multiply by 1.609344. A quick mental trick: the Fibonacci numbers approximate it — 5 mph ≈ 8 km/h, 8 mph ≈ 13 km/h.
What is a knot and how fast is it?
A knot is one nautical mile per hour, equal to 1.852 km/h or 1.151 mph. It is used in maritime and aviation contexts because a nautical mile corresponds to one minute of latitude, which simplifies chart navigation.
How do I convert km/h to m/s?
Divide by 3.6. One km/h is 0.277778 m/s, so 100 km/h is 27.78 m/s. The reverse is to multiply m/s by 3.6. This is the conversion physics and engineering problems expect.
Why does wind speed use different units in different forecasts?
Marine and aviation forecasts use knots, most national weather services use km/h, and US forecasts use mph. They all describe the same wind; only the unit differs. Converting avoids the common mistake of reading a knot value as km/h, which understates the wind by about 46%.
Is the speed of sound always 1,235 km/h?
No. At sea level on a standard day it is about 1,225 km/h (761 mph), but it drops with temperature — at typical cruise altitude it falls to roughly 1,060 km/h. The figure in the table is the sea-level value only.