Imagine a tiny ant standing at one corner of your maths book. It sets off along the edge, all the way around, until it gets back to the very corner it started from.

Hands up: how could we work out how far that ant walked on its whole trip around the edge?

The distance all the way around the edge of a shape has a name: the perimeter. To find it, we touch each side once and add the lengths together.

Watch four shapes with the length written on every side. For each one, we touch each side in turn, add as we go, and land on the total distance around.

The last one has a step in it, so it has more sides than a rectangle. Get ready to say how many sides we must add.

Today we work through four shapes together on the board: a triangle with sides 3 cm, 5 cm and 4 cm, a square with each side 5 cm, a 6 cm by 2 cm rectangle, and an L-shape. For each one, we trace every side as we count the lengths aloud, add them up, then type the perimeter and tap Check.

Now we measure real shapes for ourselves. In your groups you have a 4 cm by 6 cm card and a piece of string. Lay the string right around the edge of the card, mark where it meets, then straighten the string against your ruler to read the distance around.

Work through these in order:

• Find the perimeter of the 4 cm by 6 cm card.
• Find the perimeter of your own maths copy, to the nearest centimetre.
• Find the perimeter of the triangle card your teacher hands out.
• You have two cards. Without measuring the second one fully, decide which has the longer way around — then check.

Today you learned

• The perimeter is the distance all the way around the edge of a shape.
• To find it, touch each side once and add the lengths together.
• Never skip a side and never count a side twice.

Coming up

Next time we look more closely at rectangles, where the two long sides match and the two short sides match — so once you know two sides, you already know the other two.