Speed Converter

Every unit carries a fixed factor that converts it to metres per second, and the conversion runs in two steps: the input is multiplied by its source factor to reach metres per second, then divided by the target factor to reach the output unit. This avoids maintaining a separate rule for each of the ninety possible unit pairs.

The factors come from the standard definitions. A kilometre per hour is 1,000 divided by 3,600 metres per second, or 0.277778. A mile per hour is exactly 0.44704, following the 1959 international definition of the yard. A foot per second is exactly 0.3048. A knot is one nautical mile per hour, and since the nautical mile is exactly 1,852 metres, a knot is 1,852 divided by 3,600, or 0.514444 metres per second. A kilometre per second is 1,000 and a mile per second 1,609.344.

Mach is treated as 343 metres per second, the speed of sound in dry air at roughly 20 degrees Celsius at sea level. The speed of light is the exact defined value of 299,792,458 metres per second.

Enter 100, set the source unit to km/h and the target to mph. The first step converts to the base: 100 x 0.277778 gives 27.7778 metres per second. The second step divides by the mph factor: 27.7778 / 0.44704 gives 62.137 mph. A 100 km/h limit is therefore just over 62 mph, which is why UK drivers abroad can treat it as close to the 60 mph national limit on single carriageways.

The same input reads across the other units in one view. In knots, 27.7778 / 0.514444 is 54.0 knots. In feet per second, 27.7778 / 0.3048 is 91.13 ft/s. As a Mach number, 27.7778 / 343 is 0.081, so a motorway car travels at roughly eight percent of the speed of sound. Against light, it is 9.27 x 10 to the power of minus 8 of c.

Reversing the pair, a 70 mph limit converts to 70 x 0.44704 / 0.277778, which is 112.65 km/h.

Only the Mach figure carries real uncertainty. The speed of sound depends on air temperature and, to a lesser degree, composition, falling to roughly 295 metres per second in the cold air at cruising altitude. This tool uses a fixed 343 metres per second for 20 degrees Celsius at sea level, so treat Mach results as an approximation unless you are working at those conditions. Every other factor here is an exact definition rather than a measurement.

Do not confuse speed with pace. Runners and cyclists often think in minutes per kilometre or minutes per mile, which is the reciprocal of speed, so a straight unit conversion will not produce it; 12 km/h corresponds to a pace of 5 minutes per kilometre.

For wind and marine work, remember that knots describe speed through the water or air, not over the ground, so current and drift change the figure that matters for navigation. Rounding matters too: quote enough decimals that a converted speed limit does not creep over the legal line.