Perimeter of triangle explained

Learn about the perimeter of triangle and also learn the formula to find it. For any polygon the meaning of perimeter is same. For a circle instead of perimeter we are calling as circumference but meaning is same.

Perimeter of triangle

The perimeter of triangle is defined as “the sum of the lengths of all the sides of a triangle is called as perimeter of triangle”. To be particular we will find out perimeter formula for different types of triangles. Based on type of triangle we have to use that particular formula to find perimeter of triangle. Without formula also we can find perimeter of triangle in that case all the sides should be added.

Formula for perimeter of triangle in different cases:

Equilateral triangle:

Consider an equilateral triangle with side “P” units.

Perimeter of triangle

Perimeter of equilateral triangle is (p + p + p) = 3p units


What will be the perimeter of equilateral triangle with side 11 cm?


Perimeter of equilateral triangle

= 3p units

= 3 × 11 cm (here p = 11 cm)

= 33 cm

Isosceles triangle:

Consider an isosceles triangle with the lengths of two equal sides as “r” units and length of third side as “s” units.

Perimeter of triangle

Therefore, perimeter of isosceles triangle is (r + r + s) = 2r + s units.


Find the perimeter of isosceles triangle whose length of two equal sides is given as 5 cm and the third side is 4 cm.



r = 5 cm, s = 4 cm

Perimeter of isosceles triangle

= 2r + s units

= 2 × 5 + 4 cm

= 10 + 4 cm

= 14 cm

Scalene triangle:

Consider a scalene triangle with the lengths of three sides as “f” units, “g” units and “h” units.

Perimeter of triangle

Therefore, perimeter of scalene triangle is (f + g+ h) units.


Find out the perimeter of scalene triangle whose sides are 3 cm, 8 cm and 15 cm.



f = 3 cm, g = 8 cm and h = 15 cm.

Perimeter of scalene triangle

= f + g + h units

= 3 + 8 + 15

= 26 cm.

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