A rental car in the US shows 65 on the dashboard and the sign says 100. A treadmill quotes minutes per kilometre while your running app quotes minutes per mile. A weather forecast gives wind in knots and a cycling computer in km/h. Speed is one of the few quantities people convert almost daily, usually in their head and usually a bit wrong. This guide gives the factors, the mental shortcuts that are close enough for driving, the exact ones for anything that matters, and the pace conversions runners actually search for.
The core conversions
| From → To | Multiply by | Mental version |
|---|---|---|
| km/h → mph | 0.621371 | × 5 ÷ 8, or × 0.6 |
| mph → km/h | 1.609344 | × 8 ÷ 5, or × 1.6 |
| m/s → km/h | 3.6 | × 3.6 (exact) |
| m/s → mph | 2.23694 | × 2.2 |
| knots → km/h | 1.852 | × 1.85 (exact) |
| knots → mph | 1.15078 | × 1.15 |
| ft/s → mph | 0.681818 | × 2 ÷ 3 |
| Mach 1 (sea level, 15 °C) | ≈ 1,235 km/h ≈ 767 mph | Varies with temperature and altitude |
The mile is defined as exactly 1,609.344 metres, so km/h ↔ mph is exact, and the Speed Conversion Calculator gives as many decimals as you want without rounding drift.
Driving: the 5⁄8 rule and a lookup you can memorise
km/h to mph: multiply by 5 and divide by 8. 100 km/h → 500 ÷ 8 = 62.5 mph. mph to km/h: multiply by 8, divide by 5. 70 mph → 560 ÷ 5 = 112 km/h. For road signs, memorise a few anchors and interpolate:
- 30 km/h ≈ 19 mph (residential)
- 50 km/h ≈ 31 mph (urban default in most of the world)
- 80 km/h ≈ 50 mph
- 100 km/h ≈ 62 mph
- 120 km/h ≈ 75 mph
- 130 km/h ≈ 81 mph (French/Italian motorways)
And in reverse: 25 mph ≈ 40 km/h, 55 mph ≈ 89 km/h, 65 mph ≈ 105 km/h, 70 mph ≈ 113 km/h. The UK is the one place you’ll see mph on signs alongside a metric fuel pump.
Running and cycling pace
Runners think in pace (time per distance), not speed, and the conversion between min/km and min/mile is the one people most often get wrong because it isn’t a simple multiply: minutes per mile = minutes per km × 1.609.
| min/km | min/mile | km/h | mph |
|---|---|---|---|
| 4:00 | 6:26 | 15.0 | 9.3 |
| 4:30 | 7:15 | 13.3 | 8.3 |
| 5:00 | 8:03 | 12.0 | 7.5 |
| 5:30 | 8:51 | 10.9 | 6.8 |
| 6:00 | 9:39 | 10.0 | 6.2 |
| 7:00 | 11:16 | 8.6 | 5.3 |
Speed from pace: 60 ÷ pace in decimal minutes. A 5:30 pace is 5.5 minutes, so 60 ÷ 5.5 = 10.9 km/h. Treadmills display km/h or mph; set 10.9 for a 5:30/km run. Race predictions: a 5:00/km pace is a 25:00 5K, 50:00 10K, 1:45:30 half marathon and 3:31 marathon if held evenly, which almost nobody does.
Wind, water and air
Knots are nautical miles per hour; a nautical mile is one minute of latitude (1,852 m), which is why sailors and pilots never switched. Beaufort force is a scale, not a unit: force 4 is 11–16 knots (a “moderate breeze”), force 8 is 34–40 knots (gale). Weather apps that give wind in km/h for a country that measures in mph are converting for you; check which one you’re reading before deciding whether to sail.
Mach is a ratio to the local speed of sound, which depends on air temperature. At cruising altitude (−50 °C) Mach 1 is about 1,060 km/h, not 1,235. So “Mach 0.85” on an airliner is roughly 900 km/h true airspeed, not 1,050.
Distance, time and the third variable
Speed = distance ÷ time. Rearranged: time = distance ÷ speed, distance = speed × time. Keep the units consistent: 90 km at 120 km/h takes 0.75 h = 45 min. Mixing km with mph is the source of most “how long will it take” errors; convert speed first, then divide. The Unit Converter handles the distance side (miles ↔ km) if the trip is in one and the speed in the other.
Frequently asked questions
How do I convert km/h to mph in my head?
Multiply by 5 and divide by 8. 80 km/h → 400 ÷ 8 = 50 mph. Or just take 60%.
What is 100 km/h in mph?
62.1 mph. And 100 mph is 160.9 km/h.
How fast is a knot?
One nautical mile per hour: 1.852 km/h or 1.151 mph. 20 knots is about 37 km/h or 23 mph.
What is a 10 km/h pace in minutes per km?
6:00 per km, or 9:39 per mile. Divide 60 by the speed to get minutes per unit.
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