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

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 corner to show where you have started from to avoid missing or double counting a length.

From the diagram above: 3 + 4 + 5 + 4 + 8 + 8 = 32cm


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

Answer: 16 cm

Each side is 4cm long. 4 x 4 = 16cm

2. What is the perimeter of this triangle?

Answer: 27.31cm

The sloping side is not indicated: this has to be worked out using Pythagoras` Theorem

c2 = 82 + 82 = 128

c = 11.31

The perimeter = 8 + 8 + 11.31 = 27.31cm