📏 Geometry

Perimeter of Polygons

The perimeter is the distance all the way around a shape. Once you know how to measure the sides, finding the perimeter of any polygon is just addition.

Part 1

What Is Perimeter?

Imagine walking all the way around the edge of a shape — the total distance you'd walk is the perimeter. To find it, just add up the length of every side.

💡 Key Idea

Perimeter is measured in regular units — like inches, feet, or centimeters — not square units. That's because you're measuring a distance around the outside, not the space inside.

6 cm 4 cm 4 cm 4 cm 4 cm

Add every side: 6 + 4 + 4 + 4 + 4 = 22 cm around the outside.

Part 2

Perimeter of Rectangles

A rectangle has two pairs of equal sides — a length and a width. Instead of adding all four sides one at a time, you can double each pair and add them together.

P = 2l + 2w
l = length  •  w = width
l = 8 ft w = 5 ft

Worked Example

P = 2l + 2w
P = 2(8) + 2(5)
P = 16 + 10
P = 26 ft
Part 3

Perimeter of Squares

A square is a special rectangle where all four sides are equal. That means you only need to know one side length to find the perimeter.

P = 4s
s = length of one side
s = 7 m

Worked Example

P = 4s
P = 4(7)
P = 28 m
Part 4

Perimeter of Triangles

Triangles don't usually have equal sides, so there's no shortcut formula — just add up all three side lengths.

P = a + b + c
a, b, c = the three side lengths
9 in 7 in 11.4 in

Worked Example

P = a + b + c
P = 9 + 7 + 11.4
P = 27.4 in
Part 5

Perimeter of Other Polygons

This same rule works for any polygon, no matter how many sides it has — a pentagon, a hexagon, or an irregular shape with sides of all different lengths. Just add every side.

5 cm 6 cm 6 cm 6 cm 5 cm

A pentagon with five sides: 5 + 6 + 6 + 6 + 5 = 28 cm

💡 Key Idea

Every shape's sides must be measured in the same unit before you add them. If one side is given in inches and another in feet, convert them to match first.

Practice

Try It Yourself

Find the Perimeter

Add up the sides for each shape and type your answer. Click "New Problems" for a fresh set anytime.

Rectangle
Square
Triangle
Irregular Polygon