A perimeter is the length of the path around an area. Put another way, a shape's perimeter is the total length of its sides or outline. The perimeter of a shape is the two-dimensional equivalent of the surface area of a three-dimensional solid. You can calculate the perimeters of regular polygons by using standard formulas.

### Perimeter Formulas

You can use standard formulas to calculate the perimeters of many polygons, including the following:

Perimeter of a square = 4 * width

Perimeter of a rectangle = (2 * width) + (2 * length)

Perimeter (circumference) of a circle = pi * diameter = 2 * pi * radius

Perimeter of any regular polygon = number of sides * length of side

The perimeter of any straight-sided irregular polygon equals the sum of the lengths of the sides.

### Circumference

The perimeter of a circle is called its circumference. In measuring the perimeter of circular objects in the real world, it's usually helpful to wrap something flexible like a tape measure or piece of string around the object and find how long a piece it takes to encircle the object. The geometric formula for circumference depends on knowing the circle's diameter, the longest distance between two points on a circle's circumference, or the radius, which equals half of the diameter.

### Practical Applications

Knowing the perimeter of an object or area has many real-world applications. For example, the circumference of a car tire multiplied by the number of revolutions per minute the tire makes can help you calculate how fast the car is moving. Measuring the perimeter of various planes of your body can help you track your health or choose the right size clothing. The perimeter of a yard determines what length of fencing you would need to enclose it.

### Irregular Perimeters

Many shapes have irregular perimeters, made up of unequal angles, lines of unequal length or even curves. These perimeters can be difficult to calculate and must be approached on a case-by-case basis. If direct measurement with string or a tape measure is not an option, break down the shape into uniform curves and straight-line segments. Depending on how much information about the angles, lengths and curves you start with, you may have to use trigonometry or calculus to find the lengths of some parts of the perimeter.