Speed Converter
Convert between km/h, mph, m/s, ft/s, knots, and Mach number. Results update as you type.
Quick Reference
Understanding Speed Units
Speed measures how fast something moves — the distance traveled per unit of time. Different speed units are used in different contexts.
Common Conversion Factors
| From | To | Multiply by |
|---|---|---|
| km/h | mph | 0.621371 |
| mph | km/h | 1.60934 |
| m/s | km/h | 3.6 |
| knots | km/h | 1.852 |
| Mach 1 | km/h | 1,234.8 (at sea level) |
Speed Unit Use Cases
- km/h — Road speeds in most countries, weather reports (metric countries)
- mph — Road speeds in the US, UK, and a few other countries
- m/s — Scientific measurements, physics calculations, wind speed (SI unit)
- Knots — Maritime and aviation speed. 1 knot = 1 nautical mile per hour
- Mach — Aircraft speed relative to the speed of sound. Mach 1 — 1,234.8 km/h at sea level
Notable Speeds
| What | km/h | mph |
|---|---|---|
| Walking | 5 | 3.1 |
| Cycling | 15—25 | 9—16 |
| City driving | 50 | 31 |
| Highway driving | 100—120 | 62—75 |
| Speed of sound | 1,234.8 | 767.3 |
| Speed of light | 1,079,252,849 | 670,616,629 |
Frequently Asked Questions
How speed conversions actually work
Every speed unit reduces to a length divided by a time. Once you know the length conversion (1 km = 1000 m, 1 mile = 1609.344 m, 1 nautical mile = 1852 m, 1 foot = 0.3048 m) and the time conversion (1 hour = 3600 s), every cross-conversion is a quick fraction. The cleanest way to mentally convert km/h to m/s is to divide by 3.6 (because 3600/1000 = 3.6), and from m/s back to km/h is a multiplication by 3.6.
Where each unit is dominant
- km/h is the road-speed unit in roughly 195 of 220 countries.
- mph is dominant in the United States, the United Kingdom, and a few smaller territories.
- m/s is the SI unit, used for physics calculations, wind reporting in meteorology, and engineering.
- knots is universal in maritime and aviation because one knot equals one nautical mile per hour and one nautical mile is one minute of latitude.
- Mach describes ratios to the local speed of sound, which itself changes with air temperature and altitude.
Common confusions
Mach is not a fixed velocity. At sea level in standard atmosphere conditions Mach 1 is roughly 1235 km/h, but at 11000 m altitude where temperature is colder, Mach 1 falls to around 1062 km/h. Aircraft flight instruments calculate it from local conditions.
Knots and "knots per hour" are not the same. A knot already includes the per-hour part of the definition, so "knots per hour" actually means an acceleration. This is a frequent mix-up in casual reporting.
Worked examples
To convert 80 mph to km/h, multiply by 1.609344 to get about 128.7 km/h. To convert 45 km/h to m/s, divide by 3.6 to get about 12.5 m/s. To convert 25 knots to km/h, multiply by 1.852 to get 46.3 km/h.