Speed conversions are straightforward multiplications, but the units carry more history than most people expect — particularly the knot, which is named after actual knots in a rope.
Quick reference
The table follows whichever units you select above.
Worth memorising
1 m/s = 3.6 km/h— exact, and the easiest of all100 km/h ≈ 62 mph60 mph ≈ 97 km/h— near enough 100 for a road sign1 kn ≈ 1.15 mph ≈ 1.85 km/h
For km/h to mph in your head, take away a third and add a little back: 120 km/h minus 40 is 80, and the true answer is 75. Or halve and add ten per cent of the original: 60 plus 12 is 72, closer still.
Where the knot comes from
Sailors measured speed by throwing a weighted log overboard on a rope with knots tied at even intervals, then counting how many knots ran out in a fixed time measured by sandglass. The count was the speed, and the name stuck. Because a nautical mile is one minute of latitude, a ship making one knot covers one minute of latitude per hour — navigation arithmetic that works directly off the chart.
Aviation and shipping still use knots worldwide, alongside nautical miles for distance and feet for altitude, even in countries otherwise fully metric.
Where people go wrong
Saying "knots per hour"
A knot already means one nautical mile per hour, so "knots per hour" describes acceleration, not speed. The correct phrase is simply "twelve knots".
Pace is not speed
Runners work in minutes per kilometre or per mile, which is the inverse of speed. You cannot convert a pace by the same factor as a speed — you have to invert, convert, then invert again. A 5:00 min/km pace is 12 km/h, which is 7.46 mph, which is a 8:03 min/mile pace.
Speed limits are never converted precisely. A 30 mph limit posted in a metric country becomes 50 km/h, not 48.28. Legal limits are set as round numbers in the local unit rather than converted, so treat a converted limit as a guide and obey the posted sign.
Common questions
How fast is 100 km/h in mph?
About 62.1 mph. This is the common motorway limit across much of Europe and Australia.
What is Mach 1?
The speed of sound, which is not a fixed figure — it depends on air temperature. At sea level in standard conditions it is about 1,225 km/h or 761 mph, but at cruising altitude, where the air is far colder, it drops to roughly 1,062 km/h.
How do I convert running pace to speed?
Divide 60 by your pace in minutes per kilometre. A 6:00 min/km pace is 60 ÷ 6, so 10 km/h.