mile per hour
Speedexact by definition
The mile per hour is exactly 0.44704 m/s, following from the exact statute mile and the exact hour. It is the road speed unit of the United States and the United Kingdom, and the standard unit for wind speed in American forecasts. The factor is exact by definition.
Watch out: A mile per hour is not a knot. Twenty knots is 23 mph, so marine and aviation speeds converted as though they were road speeds come out about 15% low.
| 1 mph | 0.44704 m/s |
About the mile per hour
Miles per hour is the speed unit of two countries and a surprising number of habits elsewhere. It is exactly 0.44704 m/s, which follows from the mile being exactly 1,609.344 m and the hour being exactly 3,600 s, so the conversion involves no measurement and no rounding. The neatest identity in the unit is that 60 mph is exactly 88 ft/s, since 60 miles is 316,800 feet and an hour is 3,600 seconds. Stopping distance calculations in American driver training lean on this: at 88 ft/s, a one-second reaction time is 88 ft of road gone before the brakes touch.
Two conversions are worth carrying. To go from mph to km/h, multiply by 1.609, so 70 mph is 113 km/h and 30 mph is 48 km/h. To go from mph to m/s, divide by 2.237, or just halve and take off a bit: 60 mph is about 27 m/s. And because every force that depends on speed through the air scales with speed squared, the drag at 80 mph is not a third more than at 60 mph but nearly double, which is why fuel economy falls off a cliff above motorway speeds.