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.