How to Find the Perimeter of a Triangle

By Bonnie Crowe
A triangle is a three-sided polygon.
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The perimeter is a unit of measurement that calculates the distance around a closed shape, such as a triangle. To find the [perimeter of a triangle](http://www.mathopenref.com/triangleperimeter.html) -- assuming you know the length of the triangle's three sides -- you simply add the three sides together.

Perimeter of a Triangle

To find the perimeter of a triangle -- with sides a, b, and c -- you add the lengths of the three sides. In this example, the lengths of the sides are 4 inches, 3 inches and 4 inches, respectively. You add 4 inches + 3 inches + 4 inches. This results in a perimeter of 11 inches.

Equilateral and Isosceles Triangles

If you have an *equilateral triangle where all sides are two inches in length, you add 2 inches + 2 inches + 2 inches. This results in a perimeter of 6 inches. If you have an isosceles triangle,* two sides will have the same length, but the third will have a different length. In this example, two sides are two inches each and one side is three inches. You add 2 inches + 2 inches + 3 inches. This results in a perimeter of 7 inches.

Obtuse, Acute and Right Triangles

Whether you have an obtuse, acute or a right triangle -- the formula is the same as for any other triangle. An obtuse triangle has one internal angle greater than 90 degrees. An acute triangle has one internal angle less than 90 degrees. A right triangle has one internal angle exactly 90 degrees. To find the perimeter of any of these triangles, simply add the lengths of the three sides of the respective triangle.

About the Author

Bonnie Crowe is a mother of two teenagers; a teacher and author of children's books, curriculum and articles on English grammar, literature, technology, art, parenting and career guides for high schoolers. She's a former director of AOL Parenting, a member of SCBWI, and a graduate from the University of California,Berkeley.