The exact factor and the mental shortcut
One mile is exactly 1,609.344 meters, so one mile per hour is 1.609344 kilometers per hour, a factor derived from the NIST-published definition of the mile as 1,609.344 meters. To go from km/h to mph, multiply by 0.621371; to go the other way, multiply by 1.609344. A 100 km/h limit is 62.14 mph, and 60 mph is 96.56 km/h.
For mental math, multiply km/h by 0.6 to get mph, or mph by 1.6 to get km/h. The shortcut understates slightly — about 3 percent — which is fine for judging a sign but not for calibrating a speedometer or planning a precise arrival time.
Common limits in both units
European urban limits of 30, 50, and 60 km/h correspond to roughly 19, 31, and 37 mph. Motorway limits of 120 and 130 km/h are about 75 and 81 mph. In the other direction, US interstate limits of 65 and 75 mph are about 105 and 121 km/h, while the UK motorway limit of 70 mph is about 113 km/h.
These roundings are close enough that a driver can hold a conversation about them, but speedometers are calibrated to the local unit. A car built for mph may show km/h on an inner scale that is only approximate.
Pace versus speed
Runners usually talk about pace — minutes per kilometer or per mile — rather than speed. The two are reciprocal: a 5:00 min/km pace is 12 km/h, and a 10:00 min/mile pace is 6 mph. Converting pace is not a simple multiplication; convert the speed first, then take the reciprocal, or use a pace calculator that does it for you.
A common treadmill mistake is comparing a 10-minute mile with a 6 km/h setting. Six km/h is a 10-minute kilometer, which is a much slower pace than a 10-minute mile. Check which unit the belt displays before deciding the workout was harder than usual.
Rental cars and metric countries
When driving in a country that uses the other unit, the speedometer is usually the least of the problems. Navigation apps may announce distances in the local unit, and rental agreements may quote speed limits in one system while the dashboard shows the other.
A useful anchor pair: 50 km/h is about 31 mph, and 100 km/h is about 62 mph. Most urban driving falls between those two numbers, which makes it easy to stay roughly legal while you adjust.
A note on wind and boat speeds
Aviation and shipping use knots rather than either mph or km/h. One knot is one nautical mile per hour, about 1.852 km/h. Wind forecasts for sailors and pilots are in knots, so converting to km/h or mph requires a third factor. This is outside everyday driving but worth remembering when reading marine or aviation weather.
For road vehicles, the conversion above is all you need. Keep the exact factor for planning and the 0.6 shortcut for conversation.
Quick reference for common speeds
These are the figures drivers and runners ask about most often, converted with the exact 1.609344 factor and rounded for readability.
- 30 km/h = 18.6 mph · 50 km/h = 31.1 mph · 60 km/h = 37.3 mph
- 80 km/h = 49.7 mph · 100 km/h = 62.1 mph · 120 km/h = 74.6 mph
- 130 km/h = 80.8 mph · 140 km/h = 87.0 mph
- 30 mph = 48.3 km/h · 55 mph = 88.5 km/h · 65 mph = 104.6 km/h
- 70 mph = 112.7 km/h · 75 mph = 120.7 km/h