How to convert mph, km/h, m/s, and knots
Speed is distance over time, so converting between speed units is really just converting the distance part and keeping the time consistent. Meters per second is the physicist's base unit, and everything here normalizes through it. A kilometer per hour is 1000 meters spread over 3600 seconds, so 1 km/h = 1000/3600 ≈ 0.2778 m/s. A mile per hour uses the exact mile (1,609.344 m) over the same 3600 seconds. The structure is identical every time — only the distance factor changes.
The conversion people reach for most is between miles per hour and kilometers per hour, the two units on road signs. One mph is about 1.609 km/h, and one km/h is about 0.621 mph. So a 60 mph US highway limit is about 97 km/h, and a 100 km/h European limit is about 62 mph. If you just want a rough head-conversion, multiplying km/h by 0.6 gets you close to mph, and that's usually enough to know whether you're speeding.
Meters per second shows up in physics and engineering, and it's smaller than the road units make it feel. 1 m/s is only 3.6 km/h, a slow walk. To go from m/s to km/h you multiply by 3.6; to go back you divide by 3.6. That single factor of 3.6 is the most useful one in this whole converter for anyone doing physics homework.
The knot is the odd unit out, and it has a lovely reason for existing. One knot is one nautical mile per hour, and a nautical mile (1,852 m) was originally defined as one minute of latitude along a meridian. That ties speed directly to navigation: a ship doing one knot covers one minute of arc per hour. One knot is 1.852 km/h exactly, or about 1.151 mph. Aviation and maritime both use knots for this geographic convenience.
Worked example
A US speed limit sign reads 60 mph and you're driving a car with a km/h speedometer.
- 1 mph = 1.609344 km/h
- 60 × 1.609344 = 96.56 km/h
→ 60 mph ≈ 96.6 km/h. Keep the needle near 97 and you're at the limit.