The perimeter of a triangle is the sum of all the sides of the triangle, and you determine the perimeter of any triangle by adding the lengths of all three sides. Since an isosceles triangle has two sides of equal length, start by multiplying the length of one of the equal sides by two. Then, add the product to the length of the base side. The resulting sum is the perimeter of the triangle. Using either a pen and paper or a calculator, you can make these simple calculations in just a few minutes.
- Determine the length of the lateral sides of the triangle
An isosceles triangle has at least two equal sides. These sides are called "lateral sides." Measure the length of a lateral side using a ruler or tape measure.
- Multiply the lateral side length by 2
Since there are two lateral sides, determine the sum of the lateral sides by multiplying the length of one lateral side by 2.
- Determine the length of the base side
Measure the length of the base side of the triangle using a ruler or a tape measure. The base side is the side of the triangle with a length that is not equal to the other sides' lengths.
- Add the base length to the existing product
Add the length of the base side to the result from the second step. The sum is the perimeter of the triangle.