Speed Converter

Speed is distance covered per unit of time. The SI base is meters per second (m/s), but daily life runs on a patchwork: km/h on most road signs, mph in the US and UK, and knots at sea and in the air.

Key relationships:

• 1 km/h = 0.27778 m/s
• 1 mph = 0.44704 m/s
• 1 knot = 0.51444 m/s
• 1 ft/s = 0.3048 m/s
• 1 mph ≈ 1.609 km/h
• 1 knot ≈ 1.852 km/h

For scale:

• A brisk walk: about 5 km/h (3.1 mph)
• Highway driving: 100 km/h (62 mph)
• The speed of sound: 343 m/s (1,235 km/h, Mach 1)

The knot has a neat origin that makes it more than a nautical quirk. One nautical mile is one arcminute of latitude along a meridian, so a speed in knots maps directly onto how fast you’re crossing the globe’s grid of latitude and longitude. That’s why ships and aircraft, which navigate by that grid, never gave it up.

The conversion people get wrong most often is mph to km/h, because the 1.609 factor is close enough to “about one and a half” to invite lazy mental math. A 100 mph reading is 161 km/h, not 150, and that 11 km/h gap is exactly the sort of thing that turns into a speeding ticket when you’re driving abroad.