To properly calculate the **perimeter of a triangle** we need to know what type of triangle we have since, according to their classification, they can be:

**Equilateral triangle**: all its sides are equal and, therefore, its angles are also equal.**Isosceles triangle**two of its sides are equal while the third is different.**Scalene triangle**all its sides are different.

Depending on the case we have, **calculate the perimeter of a triangle** will be done in one way or another:

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## Perimeter of an equilateral triangle:

In the case of the equilateral triangle we have already indicated that **all sides are equal**Therefore, calculating the perimeter of this figure is as simple as adding three times the value of its side:

P = a + a + a + a = 3a

If we have a triangle whose side a = 5 centimeters, then we have that:

P = a + a + a + a = 5 + 5 + 5 + 5 = 15 centimeters.

## Perimeter of an isosceles triangle

As mentioned above, the isosceles triangle **has two equal sides** (a) and an unequal one (b), therefore, the formula for calculating the perimeter changes and becomes this:

P = a + a + a + b = 2a + b

Let's imagine that we have an isosceles triangle with side a = 6 centimeters and b = 3 centimeters. To calculate its perimeter we apply the above formula:

P = a + a + a + b = 2a + b = 6 + 6 + 3 = 15 centimeters.

## Scalene triangle

Finally, we have the case of the scalene triangle which is the one that **all sides are different**making the formula for calculating the perimeter to be:

P = a + b + c

As a practical example, suppose we have a scalene triangle with sides a = 3 cm, b = 5 cm and c = 7 cm:

P = a + b + c = 3 + 5 + 7 = 15 centimeters.

## Automatic triangle perimeter calculator

If you want to automate the process of calculating the perimeter of a triangle, you can use our calculator. Just select the type of triangle and enter how long each side is: