Converting Speed Units: mph, km/h, m/s and Knots
Speed is distance over time, which makes it the easiest conversion to reason about and the one with the most units in circulation. The Speed Converter covers metres per second, kilometres per hour, miles per hour and knots.
Which unit belongs where
km/h is road speed almost everywhere. mph is road speed in the United States and the United Kingdom - the UK being the country that buys petrol in litres and measures the journey in miles. m/s is the scientific unit, because it is what the equations of physics use. Knots are speed at sea and in the air.
The shortcuts
mph to km/h: multiply by 1.6. Thirty mph is about 48, sixty is about 97, seventy is about 113.
km/h to mph: multiply by 0.6. A hundred km/h is about 62 mph, which is why 100 and 60 feel like the same motorway limit in different countries.
m/s to km/h: multiply by 3.6, exactly. This one is worth memorising because it is exact: 10 m/s is 36 km/h.
Knots to mph: add about fifteen per cent. A hundred knots is roughly 115 mph.
A knot is a nautical mile per hour
And that is all it is - the phrase "knots per hour" is the common error, because the per hour is already inside the word. A nautical mile is 1,852 metres, defined as one minute of latitude, which is what makes it useful: sixty nautical miles is one degree of latitude on any chart, so distance and position use the same scale.
The name comes from the method. Sailors threw a log overboard on a line knotted at intervals and counted the knots that ran out in a measured time.
Where the arithmetic matters
Stopping distance grows with the square of speed. Doubling your speed roughly quadruples the distance needed to stop, which is why the gap between 30 and 40 mph is far more consequential than it sounds.
Average speed is not the average of your speeds. Drive thirty miles at 60 mph and thirty miles at 30 mph and your average is 40, not 45, because you spent twice as long on the slow half. Averaging speeds instead of dividing total distance by total time is one of the most common quiet errors in any journey calculation, and the Time Converter is often what you actually need alongside it.
Common questions
How fast is the speed of sound?
About 343 m/s at 20 Celsius, which is 1,235 km/h or 767 mph. It drops as air gets colder, which is why Mach 1 is a different speed at altitude.
Why do aircraft use knots?
Because navigation charts are marked in degrees of latitude, and a nautical mile is a minute of one. It keeps the arithmetic direct.
What is a reasonable running pace?
A ten-minute mile is 6 mph, about 9.7 km/h. Converting pace and speed is a different sum - the Running Pace Calculator does it.
Got a speed to convert?