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Perimeters of 2D Shapes

Perimeters of 2D Shapes

A perimeter is the total lengths of all the lines that form the outside of a shape.

The perimeter of a rectangle consists of adding the four lengths together: 5cm + 3cm + 5cm + 3cm = 16cm

Perimeter of a rectangle

The perimeter of a regular polygon is the number of sides multiplied by the length of one side. A regular hexagon with a side of length 5cm has a perimeter of 6 x 5 = 30cm.

When calculating the perimeter of a compound shape, determine the starting point for the shape and work round the shape in one direction to avoid leaving any sides out of the calculation. Put a mark on the starting corner to show where you have started from to avoid missing or double counting a length.

Example 1

A square is shown below. What is the perimeter of the square?

Perimeter of a square

Each side is 4cm long.

4 x 4 = 16cm

Answer: 16 cm

Example 2

What is the perimeter of this shape?

Perimeter of a Compound Shape

Missing length a = 8 - 3
= 5
Missing length b = 8 - 4
= 4
Add clockwise P = 8 + 8 + 4 + 5 + 3 + 3
= 31

Answer: 31 cm