Speed Converter

1. 1Type a value into any box — every other unit updates as you type.
2. 2Use the swap button to flip the two headline units.
3. 3Copy any result with the button beside it.
4. 4Adjust the decimal places if you need more or less precision.

Every speed is converted through metres per second, the SI base. Because the mile and the nautical mile are both exactly defined in metres, the conversions between mph, km/h and knots are exact rather than approximate.

Mach is the exception and deserves a caveat: it is a ratio to the local speed of sound, which changes with air temperature. The value used here, 340.29 m/s, is the standard sea-level figure at 15 °C. At cruising altitude the speed of sound is nearer 295 m/s, so Mach 1 there is a considerably slower ground speed.

Conversion works by taking your value to a single base unit and then out again, so every pair of units is consistent with every other. Values are calculated at full double precision and only rounded for display, which means round-tripping a number through two conversions returns what you started with.

• Use m/s for physics and engineering, km/h or mph for roads, and knots for wind, sea and air.
• To go from m/s to km/h, multiply by 3.6.
• Mach depends on air temperature; the value here is for sea level at 15 °C.
• Check which unit a speed limit sign uses when driving abroad.

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