mile
Lengthexact by definition
The statute mile is exactly 1,609.344 metres, which is 5,280 feet or 1,760 yards. It is the standard road distance unit in the United States and the United Kingdom. The factor is exact by definition, following from the exact definition of the foot.
Watch out: A statute mile is not a nautical mile. The nautical mile is 1,852 m, about 15% longer, and a speed of 20 knots is 23 mph rather than 20.
| 1 mi | 1,609.344 m |
The mile descends from the Roman mille passus, a thousand paces, where a pace was a double step of about 1.48 m. The English figure of 5,280 ft comes from reconciling the mile with the furlong of 660 ft, which was the length of a ploughed furrow: an Act of Parliament in 1593 set the mile at eight furlongs, and the awkward 5,280 has stood ever since.
About the mile
The 5,280 in a mile looks arbitrary because it is a compromise between two systems that did not want to agree. The Romans left England a mile of a thousand double paces, close to 5,000 modern feet. English farming left it the furlong, the distance a team of oxen would plough before resting, fixed at 40 rods of 16.5 ft each, or 660 ft. Land was measured in furlongs and had to keep being measured in furlongs, so when the mile was standardised the choice was to round it up to eight furlongs rather than break every existing land record. Eight times 660 is 5,280.
That decision is still visible in American land. The Public Land Survey System divides the country into townships six miles square, each holding 36 sections of one square mile, and each section is 640 acres because an acre is one furlong by one chain. Rural roads in the Midwest sit on a one-mile grid for exactly this reason, and a "quarter section" of 160 acres was the standard homestead. The number 5,280 is not a curiosity; it is the seam where a Roman unit was stitched onto a farming one.