A furlong is a measure of distance in imperial units and United States customary units equal to one eighth of a mile, equivalent to any of 660 feet, 220 yards, 40 rods, 10 chains or approximately 201 metres. It is now mostly confined to use in horse racing, where in many countries it is the standard measurement of race lengths, and agriculture, where it is used to measure rural field lengths and distances.

In the United States, some states use older definitions for surveying purposes, leading to variations in the length of the furlong of two parts per million, or about 0.4 millimetres (1⁄64 inch). This variation is too small to have practical consequences in most applications.

Using the international definition of the yard as exactly 0.9144 metres, one furlong is 201.168 metres, and five furlongs are about 1 kilometre (1.00584 km exactly).