Speed units and their equivalents
| Unit | Symbol | In meters per second |
|---|---|---|
| Kilometer per hour | km/h | 0.277778 m/s |
| Mile per hour | mph | 0.44704 m/s |
| Meter per second | m/s | 1 m/sbase unit |
| Knot | kn | 0.514444 m/s |
| Foot per second | ft/s | 0.3048 m/s |
| Kilometer per second | km/s | 1,000 m/s |
| Mile per second | mi/s | 1,609 m/s |
Frequently asked questions
Is a knot the same as an mph?
No. A knot is one nautical mile per hour, or 1.852 km/h, against 1.609 km/h for road mph. The 15% gap comes from the nautical mile, defined as one minute of latitude.
What are the speed limits abroad?
UK: 30 mph in built-up areas (48 km/h), 60 mph on single carriageways (97 km/h), 70 mph on motorways (113 km/h). US: typically 55 to 70 mph on highways, or 89 to 113 km/h depending on the state.
Is the average speed of a round trip the average of the two speeds?
No, and the gap is wide. Driving out at 60 km/h and back at 20 km/h over the same road averages 30 km/h, not 40: the slow leg takes three times as long, so it weighs three times as much. Average speed is always the total distance divided by the total time, never the mean of the speeds. The same logic holds on the motorway: covering 100 km at 130 km/h instead of 110 saves eight and a half minutes, which a single ten-minute stop wipes out.
Does driving twice as fast double the stopping distance?
No, it roughly triples it. Braking distance grows with the square of speed: on dry tarmac it goes from about 12 m at 50 km/h to 48 m at 100 km/h. On top of that comes the distance covered while reacting, which grows in proportion to speed: one second of inattention is 14 m at 50 km/h and 28 m at 100. That totals roughly 26 m against 76 m. It is also why 30 km/h in town is not half of 50: the stopping distance there falls below 13 m.
Is Mach 1 always the same speed?
No, because the speed of sound depends only on air temperature, not on pressure or on altitude as such. At sea level at 15 °C, Mach 1 is 1,225 km/h; at 11,000 m, where the air sits at −56.5 °C, it is down to 1,062 km/h. An airliner cruising at Mach 0.85 is therefore flying at 903 km/h, where the same figure at ground level would mean 1,041 km/h. A Mach number is not a speed, it is a ratio.
The metre per second rules, and no road sign shows it
The SI unit of speed is the metre per second, and not one road in the world uses it. Signs show kilometres per hour or miles per hour, physics and meteorology count in metres per second, shipping and aviation in knots. Moving between the first two is exact and easy to remember: one metre per second is exactly 3.6 kilometres per hour, since an hour holds 3600 seconds and a kilometre 1000 metres. Dividing by 3.6 is all it takes to go from the sign to the formula.
Speed is also the only quantity whose ceiling is a decided number rather than a measured one. Since 1983 the speed of light in vacuum has been 299,792,458 metres per second, exactly, because it is what defines the metre and not the other way round. No experiment can measure it any more: a finer measurement only corrects the length of the metre. That is a rare reversal in metrology, where one usually starts from the standard and arrives at the constant.