Speed Converter
Type a speed in any unit and see all the others, including the pace-per-mile and pace-per-kilometre figures runners actually use.
- 5 km in
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- 10 km in
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- Half marathon
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- Marathon
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Converting speed units
To convert km/h to mph, multiply by 0.621371; to go the other way, multiply by 1.60934. A useful mental shortcut is that mph is roughly five-eighths of km/h: 80 km/h is about 50 mph, and 100 km/h is about 62 mph. A knot is one nautical mile per hour, defined as exactly 1.852 km/h, which is why aviation and shipping speeds always look slightly higher than the equivalent in mph.
Speed reference
| km/h | mph | m/s | Roughly |
|---|---|---|---|
| 5 | 3.1 | 1.39 | Walking pace |
| 10 | 6.2 | 2.78 | Steady jog, 6:00/km |
| 20 | 12.4 | 5.56 | Casual cycling |
| 50 | 31.1 | 13.9 | Urban speed limit |
| 100 | 62.1 | 27.8 | Motorway cruising |
| 120 | 74.6 | 33.3 | European motorway limit |
| 1,235 | 767 | 343 | Speed of sound at sea level |
Speed versus pace
Runners think in pace — minutes per kilometre or mile — while cyclists and drivers think in speed. They are inverses of each other, which is why a small pace change feels much larger at faster speeds.
| Pace per km | Speed | 5 km | Marathon |
|---|---|---|---|
| 7:00 | 8.6 km/h | 35:00 | 4:55:22 |
| 6:00 | 10.0 km/h | 30:00 | 4:13:11 |
| 5:00 | 12.0 km/h | 25:00 | 3:30:59 |
| 4:00 | 15.0 km/h | 20:00 | 2:48:47 |
The race predictions above assume you hold the same pace throughout, which almost nobody does over a marathon. Real marathon times are typically several percent slower than a pace held for 5 km would suggest.
How to use the Speed Converter
- Type a speed into whichever unit you know.
- Every other unit updates instantly.
- The pace section converts the same speed into minutes per kilometre and per mile, with predicted race times.
Frequently asked questions
How fast is 100 km/h in mph?
62.1 mph. The quick approximation is to take 60% and add a bit — 60 mph gets you close enough for reading a speedometer abroad.
What is a knot?
One nautical mile per hour, defined as exactly 1.852 km/h or about 1.151 mph. It is used in shipping and aviation because a nautical mile corresponds to one minute of latitude, which makes navigation arithmetic simpler.
How do I convert speed to running pace?
Divide the distance by the speed. At 10 km/h, one kilometre takes 0.1 hours, which is 6 minutes — a 6:00/km pace. The tool does this for both kilometres and miles automatically.
Why do the race predictions look optimistic?
Because they assume perfectly even pacing over the whole distance. In practice fatigue means longer races are run slower per kilometre, so treat the marathon figure especially as a theoretical ceiling.
Is Mach a fixed speed?
No. Mach 1 is the local speed of sound, which falls with temperature. It is about 1,235 km/h at sea level but closer to 1,062 km/h at cruising altitude, which is why aircraft speeds are quoted as Mach numbers rather than fixed values.
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Last reviewed September 4, 2026. Results are estimates for general information only.