# Length

Length usually refers to the measure of the distance from one end of any object or space to the other. It is a measurement in one dimension, where area is a measurement in two dimensions, and volume is a measurement in three dimensions.

## Units of length

There are many units that are used to measure length. In the most widely adopted International System of Units (SI), the base unit of length is the meter. In SI, kilometer, centimeter, millimeter, micrometer, among others are derived from meter and are used often in different situations. In the US, the lengths used in most everyday situations are the inch, foot, yard, and mile. The US also uses SI in various areas such as science, industry, the government, and more.

## Terms used to describe fixed lengths

There are a number terms used to describe the lengths in a fixed object. Some of the more common terms are provided below:

- Length - This is often used alongside width. In geometric contexts, when a length and a width are involved (such as with a rectangle), length is usually considered the longer of the two dimensions.
- Width - This is usually the shorter length, where the longest length is referred to as "length." The term breadth may also used.
- Height - This is used to measure vertical length from the base of an object, such as a triangle.
- Depth - In three dimensional objects, depth usually refers to the third dimension, where the first two may be height and width.
- Perimeter - the sum of the lengths of the sides of a polygon.
- Circumference - the length of the boundary of a circle or circular object.

### Did you know?

A boa constrictor can grow to a length of 10 to 15 feet.