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.

Pace is speed upside down

Runners, rowers and swimmers do not think in speed at all. They think in pace: minutes per kilometre or minutes per mile, which is time divided by distance rather than distance divided by time. The two are reciprocals, so as one goes up the other goes down, and that inversion is why pace arithmetic feels awkward.

Pace in min/km = 60 ÷ speed in km/h

A 12 km/h runner is doing 5:00 per kilometre, or 8:03 per mile. Drop to 10 km/h and the pace becomes 6:00 per kilometre. The calculator prints both alongside every conversion, because a treadmill shows km/h while a race result shows pace and nobody enjoys doing that division mid-run.

Mach is not a fixed speed

Mach 1 is the local speed of sound, and the speed of sound depends on the temperature of the air, not on pressure or altitude directly. It is 343 m/s at 20°C at sea level, but only about 295 m/s at the −50°C an airliner cruises in. So an aircraft at Mach 0.85 is doing roughly 900 km/h up there and would be doing 1,050 km/h at the same Mach number near the ground. The Mach figure this page prints uses the sea-level 343 m/s value.