Enter a speed in any of the seven fields and the other six follow as you type. Mach uses a fixed 340.3 m/s instead of a temperature-corrected value.
Runs locally in your browserType a speed into any of the seven fields and the other six follow as you edit; there is no Convert button, and the field you are typing in is never reformatted under your cursor. The rows cover metres per second, kilometres per hour, inches per second, kilometres per second, the speed of light, Mach and miles per hour.
The conversion runs in this browser. No value is sent to a server, the page keeps working offline, and nothing you type is stored.
Every row is converted through metres per second. 1 km/h is exactly 1/3.6 m/s, 1 in/s is exactly 0.0254 m/s on the international inch, 1 km/s is 1,000 m/s, and 1 mph is exactly 0.44704 m/s because the international mile is 1609.344 m. The light-speed row uses the defined value c = 299,792,458 m/s, so 1 c reads back as 299,792,458 m/s.
The Mach row is the exception: Mach 1 has no single value, because the speed of sound depends on temperature and on the medium. This page uses a fixed 340.3 m/s - Mach 1 in dry air at 15 °C at sea level - and it does not adjust for altitude, temperature or a different gas. Treat that row as an approximation.
Results carry up to 15 significant digits and switch to exponent notation below 0.000001 and at 10^15 and above, so 1 m/s reads as 3.33564095198152e-9 c instead of a row of zeros. Integers up to 9,007,199,254,740,991 (2^53 - 1) convert exactly; a longer number is rounded by the browser to the nearest value it can hold, and the page says so under the fields.
The row you type in keeps exactly what you wrote; only the other six are rewritten. Because each row keeps 15 significant digits, a converted result read back through a second row can land one unit in the last digit away from the number you started with - measured, for example, when 1 m/s comes back from the mph row as 0.999999999999999.
There is no knot, foot per second or furlong row, no temperature-corrected Mach, and no acceleration, distance or time calculation: the page converts the seven speeds above and nothing else.
It also does not read a speed out of a GPS log or an odometer, does not round to a chosen number of digits, and writes results without thousands separators so that any of them can be typed back into another row.