Imagine a garden shaped like a pentagon, with five straight sides: 3 m, 5 m, 4 m, 6 m and 7 m. To build a fence all the way around it, how much fencing would you need?

Watch as we walk the boundary of three rectangles on the board. Each one shows the perimeter readout at the bottom: the running total of all four sides.

6 m × 4 m rectangle

A long-side, short-side, long-side, short-side trip round the edge: 6 + 4 + 6 + 4 = 20 m.

5 m × 5 m square

All four sides are the same: 5 + 5 + 5 + 5 = 20 m. Same perimeter as the first one, but a very different shape.

8 m × 3 m long rectangle

Stretched out: 8 + 3 + 8 + 3 = 22 m. Long and thin uses up more fencing than tall-and-square does for the same floor space.

A five-sided shape

Now a shape that isn't a rectangle. We do exactly the same thing — add up every side: 6 + 4 + 5 + 5 + 4 = 24 m.

An L-shape

This one has six sides. Walk all the way round and add every one: 3 + 3 + 4 + 2 + 7 + 5 = 24 m. The same trick works no matter how many sides a shape has.

We're going to measure three small objects in the room, all in centimetres: the Maths book, an A4 page from your folder, and a copybook. One volunteer at a time comes up to measure with the tape; another pupil enters the dimensions on the board. The class predicts the perimeter before the readout shows it.