Speed Converter

The everyday conversions

One mile per hour is exactly 1.609344 kilometres per hour, because the mile is defined exactly in metric terms. One metre per second is 3.6 km/h exactly, which makes it the easiest mental conversion in the set. A knot is one nautical mile per hour, exactly 1.852 km/h.

Why knots exist

A nautical mile was defined as one minute of latitude along a meridian, which ties it directly to navigation: a degree of latitude is sixty nautical miles anywhere on Earth. That relationship is why aviation and shipping still use knots rather than switching to metric. It is not tradition for its own sake; the unit does arithmetic on a chart that kilometres cannot.

Mach is not a fixed speed

Mach 1 is the local speed of sound, which depends on air temperature, not altitude directly. At sea level in standard conditions it is about 1,235 km/h; in the stratosphere at minus 57 Celsius it drops to roughly 1,062 km/h. An aircraft can accelerate through Mach 1 while flying at constant speed, simply by climbing into colder air. Any single figure quoted for Mach 1 carries an assumed temperature.

Pace is the inverse of speed

Runners and cyclists work in minutes per kilometre or per mile, which is time divided by distance rather than distance divided by time. Converting requires inverting, not scaling, and this catches people: halving your pace doubles your speed. A 5:00 per kilometre pace is 12 km/h, and a 4:00 pace is 15 km/h, so a one-minute improvement is worth 3 km/h at that end of the range and less at slower paces.

Where errors have consequences

Speed limits posted in one unit and read in another are a routine cause of foreign-driving fines: 100 read as mph in a 100 km/h zone is nearly 60 percent over. Aviation has had incidents from mixed units in flight planning. State the unit explicitly at every interface rather than assuming a regional default.