The definitions behind the numbers
Appendix B.8 of the NIST Guide to the SI lists these conversion factors as exact:
- 1 mile (international) = 1.609344 km (exact)
- 1 nautical mile = 1852 m (exact)
- 1 mph = 1.609344 km/h = 0.44704 m/s (both exact)
A knot is one nautical mile per hour, so 1 knot = 1.852 km/h. And since an hour has 3600 seconds and a kilometer has 1000 meters, km/h is m/s times 3.6.
Conversion formulas
| Convert | Formula |
|---|---|
| km/h → mph | km/h ÷ 1.609344 |
| mph → km/h | mph × 1.609344 |
| km/h → knots | km/h ÷ 1.852 |
| knots → km/h | knots × 1.852 |
| m/s → km/h | m/s × 3.6 |
| km/h → m/s | km/h ÷ 3.6 |
For mental math: mph is roughly 0.6 of km/h, and knots are a little over half of km/h.
Chart: km/h to mph, knots and m/s
Values are rounded to one decimal place.
| km/h | mph | knots | m/s |
|---|---|---|---|
| 10 | 6.2 | 5.4 | 2.8 |
| 20 | 12.4 | 10.8 | 5.6 |
| 30 | 18.6 | 16.2 | 8.3 |
| 40 | 24.9 | 21.6 | 11.1 |
| 50 | 31.1 | 27.0 | 13.9 |
| 60 | 37.3 | 32.4 | 16.7 |
| 80 | 49.7 | 43.2 | 22.2 |
| 100 | 62.1 | 54.0 | 27.8 |
| 120 | 74.6 | 64.8 | 33.3 |
Chart: mph to km/h
| mph | km/h |
|---|---|
| 10 | 16.1 |
| 20 | 32.2 |
| 30 | 48.3 |
| 40 | 64.4 |
| 50 | 80.5 |
| 60 | 96.6 |
| 70 | 112.7 |
Knots on the water
Boats and aircraft measure speed in knots. With 1 knot = 1.852 km/h, 10 knots is about 18.5 km/h, 20 knots about 37.0 km/h and 30 knots about 55.6 km/h. If you are watching GPS speed in km/h on a ferry deck, divide by 1.852 to get knots.
Running pace
Runners often think in minutes per kilometer or per mile. Since an hour has 60 minutes:
Pace (min/km) = 60 ÷ speed (km/h), and pace (min/mile) = 60 ÷ speed (mph)
Multiply the decimal part by 60 to get seconds (for example, 60 ÷ 8 = 7.5 minutes = 7:30).
| Speed (km/h) | Pace (min/km) |
|---|---|
| 6 | 10:00 |
| 8 | 7:30 |
| 10 | 6:00 |
| 12 | 5:00 |
| 15 | 4:00 |
| 20 | 3:00 |
To go the other way, speed = 60 ÷ pace in minutes. A 5:30 per km pace is 60 ÷ 5.5, or about 10.9 km/h.
A note on converting GPS speed
The conversions are exact, but the GPS speed you start from is not. According to GPS.gov, smartphone GPS is typically accurate to within 4.9 m (16 ft) under open sky, worse among buildings and trees, and it stops in tunnels. At walking or slow running speeds the number wobbles, so judge your pace from the average over a decent distance rather than the instantaneous speed. More in how accurate is GPS speed.
Whatever the unit, the driver or rider does not look at a phone while moving. Follow local traffic laws; in Japan, Article 71, item 5-5 of the Road Traffic Act prohibits it for cars, mopeds and bicycles alike, except when stopped.