Measurement, area & perimeter
Perimeter
The perimeter of a shape is the distance all the way around its edge. You find it by adding up the lengths of all the sides.
What perimeter means
The perimeter of a shape is the total length of all of its sides. Imagine an ant walking all the way around the edge of the shape and back to where it started — the distance it walks is the perimeter.
Finding perimeter by adding sides
To find the perimeter, add every side once as you go around. A square with sides of 3 cm has 4 sides: 3 + 3 + 3 + 3 = 12 cm. A rectangle 7 cm by 3 cm has two long and two short sides: 7 + 3 + 7 + 3 = 20 cm.
Regular and irregular shapes
A regular polygon has all its sides and angles the same size, like a square. An irregular polygon has sides and angles of different sizes. You still add all the sides to find the perimeter.
Worked examples
Perimeter of a square
- The square has sides of 3 cm.
- A square has 4 equal sides.
- Add them all: 3 + 3 + 3 + 3.
Answer: The perimeter is 12 cm.
Perimeter of a rectangle
- The rectangle is 7 cm long and 3 cm wide.
- It has two long sides and two short sides.
- Add all four sides: 7 + 3 + 7 + 3.
Answer: The perimeter is 20 cm.
Perimeter of an irregular shape
- The shape has sides of 4 cm, 2 cm, 3 cm, 1 cm, 1 cm and 3 cm.
- Go around the edge, adding each side once.
- Add them all: 4 + 2 + 3 + 1 + 1 + 3.
Answer: The perimeter is 14 cm.
Where you see this in real life
- Putting a fence all the way around a garden.
- Measuring ribbon to go around the edge of a picture.
- Walking all the way around the edge of a playground.
Key words
- Perimeter
- The total length all the way around the edge of a shape.
- Polygon
- A flat shape with straight sides.
- Regular shape
- A shape with all sides and angles the same size.
- Irregular shape
- A shape with sides and angles of different sizes.
- Side
- One of the straight edges of a shape.