How the speed units connect
Speed is distance divided by time, so a speed conversion is really a length conversion with the clock held fixed. Because a mile is exactly 1,609.344 meters, one mile per hour is exactly 0.44704 meters per second and 1.609344 kilometers per hour. To go from mph to km/h multiply by 1.609344; to go back, multiply by 0.621371. Feet per second is handy in physics and ballistics: one mph is 1.46667 ft/s, so a 60 mph car covers 88 feet every second, which is roughly a tenth of the length of a football field.
A knot is one nautical mile per hour, and since a nautical mile is exactly 1,852 meters, a knot is 1.15078 mph or 1.852 km/h. Ships, aircraft and hurricane forecasts all use knots because a nautical mile matches one minute of latitude, which keeps navigation charts simple. Mach number is a ratio rather than a fixed speed: it compares an object to the local speed of sound, which falls as air gets colder. This converter uses the standard sea-level value of 340.29 m/s, so Mach 1 is 761.2 mph or 1,225 km/h; at 35,000 feet the same Mach 1 is only about 660 mph.
Worked examples
Driving: a 60 mph limit is 96.6 km/h, and the common European 100 km/h sign is 62.1 mph. A 130 km/h autobahn stretch is 80.8 mph. Rule of thumb: add half again plus a tenth to get km/h from mph, so 40 mph is about 40 + 20 + 4 = 64 km/h, and the exact figure is 64.4.
Running: 6 mph is a 10:00 mile and 9.66 km/h, which finishes a marathon in 4.37 hours. An 8:00 mile pace is 7.5 mph or 12.07 km/h. Elite marathoners hold about 13 mph, a 4:36 mile pace. Sports and weather: a 95 mph fastball is 152.9 km/h and 42.5 m/s; a Category 3 hurricane at 115 mph is 100 knots; Usain Bolt peaked near 27.8 mph, which is 12.4 m/s.
Pace, averages and caveats
Runners and cyclists usually think in pace, the inverse of speed, expressed as minutes per mile or per kilometer. Because it is an inverse, you cannot average paces directly: two miles at 8:00 and one mile at 10:00 is not a 9:00 average, it is total time divided by total distance, which is 8:40. The converter shows both a per-mile and a per-kilometer pace for any speed you enter, so you can move between a treadmill in mph and a race split in minutes per kilometer.
Remember that speedometers read a few percent high by design, and that GPS watches drift under trees and in cities. When you need a precise average speed, use measured distance over elapsed time rather than an instantaneous readout. For wind, the National Weather Service reports sustained speed in mph for land forecasts and knots for marine forecasts, so the same storm can appear with two very different numbers.