The SI base unit of length, defined by the speed of light in a vacuum.
Kilometer (km)
1,000 meters; used for road distances and geographic measurements.
Inch (in)
An imperial unit equal to 2.54 cm; used for screen sizes, hardware, and body measurements in the US.
Foot (ft)
12 inches or 0.3048 meters; the primary unit for building dimensions in the US.
Mile (mi)
5,280 feet or 1.609 km; the standard unit for road distances in the US and UK.
Real-World Examples
Example 1
Marathon Distance
26.2 miles
42.16 km — the official marathon distance
Example 2
Ceiling Height
9 ft
2.7432 m — standard ceiling height in modern US construction
Common Length Equivalents
Measurement
Metric
Imperial
Human height
175 cm
5 ft 9 in
Football field
91.44 m
100 yards
City block
~100 m
~328 ft
1 mile
1.609 km
5,280 ft
1 meter
100 cm
3 ft 3.37 in
The History and Practice of Length Measurement
From Body Parts to Light Speed
Ancient length units were based on body parts—the cubit (forearm), the foot, and the span (outstretched hand). The metric system, introduced in 1799, tied the meter to the Earth's meridian. Today, the meter is defined by the distance light travels in 1/299,792,458 of a second, making it the most precisely defined length unit in history.
Metric Adoption Worldwide
All but three countries—the United States, Liberia, and Myanmar—have officially adopted the metric system for everyday use. Even in the US, science, medicine, and the military use metric units. Understanding both systems remains essential for international commerce, engineering specifications, and academic research.
Avoiding Common Conversion Errors
When converting between systems, watch for the distinction between US survey feet (used in older land records) and international feet. Also note that nautical miles (1,852 m) differ from statute miles (1,609 m). Always confirm which unit system a measurement uses before converting.