Imagine a tiny ant standing at one corner of the picture frame at the front of the room. The ant decides to walk all the way around the very outside edge and stop back where it started. How could we work out exactly how far that little ant has travelled?

Four shapes are on the grid, each with its sides already added up. See how we go right around the outside and count each side once. That total distance around the shape is called the perimeter.

Three shapes are on the grid. One pupil at a time comes to the board, points at a shape, and says each side out loud going right around the outside while the class adds them up together.

Say the total first. Then read the P beside the shape to check you agree with it.

When a shape is done, drag one of its corners to make it a different size and do it again, so the numbers keep changing.

Key point Go right around the outside until you land back where you started, and touch every side once and only once.

Today we measure real flat objects around the room with a ruler. For each object, measure every side, add them up, and find the perimeter in centimetres. Work right around so no side is missed. Measure as many as you can, starting with one.

• your copybook cover
• a desk-name card
• a playing card
• a small square of card, about the size of a drinks coaster
• a triangle from the shape tray

What we learned today

• Perimeter is the whole distance around the outside of a flat shape.
• We find it by adding the length of every side.
• We touch each side once and only once, going right around the shape.
• We can measure the perimeter of real flat objects with a ruler.