How to calculate the perimeter of a triangle

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 triangletwo of its sides are equal while the third is different.
  • Scalene triangleall its sides are different.

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

Perimeter of an equilateral triangle:

Equilateral triangle

In the case of the equilateral triangle we have already indicated that all sides are equalTherefore, 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

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

Scalene triangle

Finally, we have the case of the scalene triangle which is the one that all sides are differentmaking 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

Perimeter of a triangle

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:

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