lengths

Length is the measurement of the extent of an object or the distance between two points along a straight line. In physics and everyday use, length refers to the linear dimension; in mathematics, arc length measures the length of a curve.

The SI base unit for length is the meter. Since 1983, the meter is defined by the distance light travels in vacuum in 1/299,792,458 of a second. The meter is used with prefixes to form kilometers, centimeters, millimeters, and so on. Common prefixes include kilo (10^3), centi (10^-2), milli (10^-3), and micro (10^-6). Non-SI units include inch (2.54 cm exactly), foot (0.3048 m), yard (0.9144 m), mile (1,609.344 m), and nautical mile.

Measurement practices rely on tools such as rulers, tape measures, calipers, micrometers, and laser rangefinders. Measurements have uncertainty, and significant figures express the precision of a result. In surveying, lengths may be measured with chains, tapes, or electronic devices.

Lengths support many activities, including geometry, construction, manufacturing, and navigation. Distances can be the straight-line distance between points or the length of a curve (arc length). In science, length scales range from molecular to astronomical.

Standards organizations publish definitions, reference artifacts, and calibration services to ensure consistency. The history of length includes artifacts such as standard rods and, in modern times, atomic definitions that rely on reproducible physical constants.