Speed Converter

Most speed units are simply a distance over a time and convert by a fixed factor. Two of the ones on this page don't behave that way, and both appear often enough to be worth understanding.

A knot is tied to the shape of the Earth

A knot is one nautical mile per hour, and the nautical mile isn't an arbitrary length. It was defined as the distance covered by one minute of latitude, and is now fixed at exactly 1,852 meters. That origin is the reason aviation and marine navigation still use it: with charts marked in degrees and minutes, a speed in knots converts directly into position change without an extra conversion step.

The name is a genuine artifact of how speed used to be measured. A line with knots tied at intervals was let out behind a ship and the knots counted against a sand timer, which is where both the unit and the term originate.

Mach isn't a speed

Mach expresses speed as a ratio to the local speed of sound, and the speed of sound varies with the temperature of the air. It falls as air gets colder, which means it falls with altitude through most of the atmosphere. The practical consequence is that Mach 1 near the ground is meaningfully faster in absolute terms than Mach 1 at cruising altitude.

Any conversion from Mach to a fixed speed therefore carries an assumption about conditions, and a figure quoted without stating altitude and temperature is incomplete. This is also why aircraft switch between indicated airspeed at low altitude and Mach number at high altitude, since the limiting factor changes.

Pace is the inverse of speed

Runners, cyclists and swimmers usually work in pace, meaning time per unit distance, such as minutes per kilometer. Speed is distance per unit time. They describe the same motion inverted, which produces a counterintuitive property: a lower pace is faster, while a lower speed is slower.

Inversion also means the two don't average the same way. Averaging pace across segments and averaging speed across the same segments give different answers, and the speed average is the one that corresponds to total distance over total time. If a training figure looks wrong, this is usually why.