The metre is the SI base unit of length, defined as the distance light travels in a vacuum in exactly 1/299,792,458 of a second. Two major systems coexist worldwide: the metric system (using powers of ten — mm, cm, m, km) adopted globally in science and everyday life, and the imperial/US customary system (inches, feet, yards, miles) primarily used in the United States. Engineers specify manufacturing tolerances in millimetres; astronomers measure cosmic distances in light-years (≈ 9.46 × 10¹⁵ m).
Imperial length ratios follow historical conventions: 12 inches = 1 foot, 3 feet = 1 yard, 1,760 yards = 1 mile. These non-decimal relationships make arithmetic harder than metric — a key reason most nations adopted SI units. The international inch is exactly 25.4 mm, making all imperial-to-metric conversions exact by definition.
| 1 inch | = 2.54 cm |
| 1 foot | = 30.48 cm |
| 1 yard | = 0.9144 m |
| 1 mile | = 1.60934 km |
| 1 metre | = 3.28084 ft |
| 1 km | = 0.621371 mi |