Speed Calculator
Solve for speed, distance, or time · mph, km/h, m/s, knots
Quick answer: 100 km/h = 62.1 mph = 27.8 m/s = 54.0 knots. Cruising at 60 mph for 2.5 hours covers exactly 150 miles (241 km).
Speed Limits by Country
| Country | Unit | Urban | Motorway |
|---|---|---|---|
| 🇺🇸 USA | mph | 25-35 | 65-70 (state varies) |
| 🇬🇧 UK | mph | 30 | 70 |
| 🇩🇪 Germany | km/h | 50 | None (advisory 130) |
| 🇫🇷 France | km/h | 50 | 130 |
| 🇦🇺 Australia | km/h | 50-60 | 110-130 |
| 🇨🇦 Canada | km/h | 50 | 100-110 |
| 🇯🇵 Japan | km/h | 40-60 | 100-120 |
| 🇮🇳 India | km/h | 50 | 100-120 |
| 🇧🇷 Brazil | km/h | 50-60 | 110-120 |
Why do only the US and UK use mph?
The UK and USA are the only major countries that use miles per hour for road speeds. The UK is officially metric for most measurements but legally required to use miles on road signs. The USA retained the imperial system largely due to infrastructure costs of changing signage — an estimated $500M to switch all road signs to metric.
Knots are used worldwide in aviation and maritime navigation because 1 nautical mile = 1 minute of arc of latitude, making navigation calculations much simpler.
Frequently Asked Questions
What is the speed of sound in km/h?
The speed of sound in dry air at 20°C (68°F) is 343 m/s = 1,235 km/h = 767 mph = 667 knots. This is Mach 1. Aircraft are considered supersonic above Mach 1. The speed of sound varies with temperature and altitude — decreasing as altitude increases.
How fast is light and can we measure it in km/h?
The speed of light in a vacuum is 299,792,458 m/s = 1,079,252,849 km/h ≈ 670,616,629 mph. It is the fundamental constant of the universe (c). GPS satellites must account for relativistic effects because light-speed signals take time to travel and clocks on the satellite tick at different rates.
What is the difference between velocity and speed?
Speed is a scalar quantity — it only has magnitude. Velocity is a vector — it has both magnitude and direction. A car driving in a circle at a constant 60 km/h has constant speed but constantly changing velocity. Speedometers measure speed; radar guns measure velocity components.