A kilometer (km), or kilometre, is a unit of length in the metric system of measurement. It is one of the most common measurements of land distance used around the world, with the exception of the US and the UK. While both may use kilometers to an extent, miles are more commonly used to reference larger land distances.

Below are some comparisons between distances in kilometers and other common measurements of length:

1 kilometer ≈ 0.621 miles

1 kilometer ≈ 3281 feet

1 kilometer ≈ 1094 yards

Kilometers and SI prefixes

SI stands for the International System of Units. The SI base unit of length is the meter, or metre. A kilometer measures the same physical quantity as a meter, except that it expresses a larger quantity than a meter, indicated by the prefix "kilo-", which stands for a factor of 1000. This means:

1 kilometer = 1000 meters

The use of prefixes to indicate either smaller or larger units of the same quantity is used throughout SI. Other examples include the centimeter, millimeter, kilogram, gram, millisecond, and more. Refer to the metric system to view more about SI prefixes.

Essentially, each prefix indicates some power of 10 by which the base unit is multiplied or divided. In the case of the kilometer, 1 meter is multiplied by 1000 or 103.