Five units of distance over time
A speed unit is a length unit divided by a time unit, so converting speed means converting the distance part and the time part. The SI unit is the meter per second. Everyday units swap in longer distances and the hour:
- Kilometers per hour (km/h): road speeds in most of the world. 1 km/h is 1,000 m in 3,600 s, or exactly 1/3.6 m/s.
- Miles per hour (mph): road speeds in the United States and the United Kingdom. Since the mile is 1,609.344 m, 1 mph is exactly 1.609344 km/h.
- Meters per second (m/s): physics, sport science and wind reports from some national weather services.
- Knots (kn): one nautical mile (1,852 m) per hour, exactly 1.852 km/h. Ships and aircraft navigate in nautical miles because one nautical mile corresponds closely to one minute of latitude, so speeds stay in knots too.
- Feet per second (ft/s): US engineering, ballistics and some sports measurements.
The converter turns your value into meters per second and then into the unit in the To select, so every pair uses exact factors.
Speed formulas
All four factors are exact. Two quick checks: 60 mph is exactly 88 ft/s, and 1 m/s is exactly 3.6 km/h.
For mental arithmetic, multiplying km/h by 5/8 gives mph about 0.6% too high: 100 km/h becomes 62.5 mph against the exact 62.14 mph. Going the other way, multiply mph by 8/5.
Feet per second and meters per second are useful when you want to picture distance covered in a moment rather than in an hour. At 60 mph a vehicle travels 88 ft every second, and at 90 km/h it covers 25 m every second, which is why a one-second glance away from the road at those speeds means traveling the length of several cars without looking.
Worked examples
- Divide by the length of a mile in kilometers: 100 ÷ 1.609344 = 62.137 mph.
- Speedometers round, so 62 mph is the practical reading.
- Multiply by 1.609344: 70 × 1.609344 = 112.654 km/h.
- A knot is exactly 1.852 km/h: 20 × 1.852 = 37.04 km/h.
- For mph, a knot is 1.150779 mph: 20 × 1.150779 = 23.016 mph.
- Multiply by 3.6: 10 × 3.6 = 36 km/h.
- One meter per second is 2.236936 mph, so 10 m/s is 22.37 mph.
Speed tables
Common round numbers on metric speed signs, converted to mph. These are conversions only; actual limits depend on the country, the road and the vehicle.
| Kilometers per hour (km/h) | Miles per hour (mph) | Meters per second (m/s) |
|---|---|---|
| 20 | 12.427 | 5.5556 |
| 30 | 18.641 | 8.3333 |
| 40 | 24.855 | 11.111 |
| 50 | 31.069 | 13.889 |
| 60 | 37.282 | 16.667 |
| 70 | 43.496 | 19.444 |
| 80 | 49.71 | 22.222 |
| 90 | 55.923 | 25 |
| 100 | 62.137 | 27.778 |
| 110 | 68.351 | 30.556 |
| 120 | 74.565 | 33.333 |
| 130 | 80.778 | 36.111 |
| Miles per hour (mph) | Kilometers per hour (km/h) | Knots (kn) |
|---|---|---|
| 10 | 16.093 | 8.6898 |
| 20 | 32.187 | 17.38 |
| 25 | 40.234 | 21.724 |
| 30 | 48.28 | 26.069 |
| 40 | 64.374 | 34.759 |
| 50 | 80.467 | 43.449 |
| 60 | 96.561 | 52.139 |
| 70 | 112.65 | 60.828 |
| 80 | 128.75 | 69.518 |
Speed conversion mistakes
- Treating a knot as a land mile per hour. A knot is about 15% faster than 1 mph. A 20-knot ferry is doing 23 mph, not 20.
- Writing “kph” and “km/h” as different units. They mean the same thing; km/h is the standard symbol.
- Dividing by 3.6 in the wrong direction. m/s to km/h multiplies by 3.6, so the km/h figure is always the bigger number. If 10 m/s comes out as 2.78 km/h, the operation was reversed.
- Rounding the factor to 1.6 for long distances or high speeds. At 130 km/h, 1.6 gives 81.25 mph against 80.78 mph; on a speed sign that half-mile-per-hour gap can matter.
Pace (minutes per kilometer or per mile) is the reciprocal of speed and cannot be entered here. Convert pace to a speed first, or use the speed, distance and time calculator.
Questions people ask
Is 100 km/h the same as 60 mph?
Close, but not equal. 100 km/h is 62.1 mph, and 60 mph is 96.6 km/h.
Why do pilots and sailors use knots?
A nautical mile is close to one minute of latitude, which makes distances on navigation charts easy to measure. Speeds in knots then match those chart distances directly.
How do I turn km/h into m/s without a calculator?
Divide by 3.6. A quick route is to divide by 4 and add a tenth of the answer back: 90 km/h ÷ 4 = 22.5, plus 2.25 gives 24.75 against the exact 25 m/s.
How many knots is 100 km/h?
About 54 knots (53.996), because each knot is 1.852 km/h. The same speed is 62.1 mph, so knots always give the smallest of the three numbers.
Sources
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B, National Institute of Standards and Technology.
- The International System of Units (SI Brochure), 9th edition, Bureau International des Poids et Mesures.