Here is a rectangle that is 5 cm along the top and 2 cm down the side. Look at it carefully. Are any of its sides the same length as each other? Which ones? Do we really need to measure all four sides to find the distance all the way around, or could we be a bit cleverer about it?

The perimeter of a shape is the distance all the way around its edge, found by adding the length of every side. The space inside the shape is its area. Today we work on perimeter.

The interactive shows three shapes in turn: a rectangle, a square and a regular pentagon. Watch how we find the perimeter of each, and look for a quicker way than adding every side one at a time.

Here is a 6 cm by 3 cm rectangle on the board. Let's find its perimeter the smart way together. First, tell me which sides are equal. Then, instead of adding all four sides one slow side at a time, pair the equal sides: add one long side and one short side, then double it. Work it out before we check.

On the board is a 4 cm by 4 cm square. Spot the equal sides first, then find its perimeter the quickest way you can. After that, I will draw three more shapes for us to work the same way: a 10 cm by 6 cm rectangle, a regular hexagon with sides of 5 cm, and a regular octagon with sides of 7 cm. For each one, name the equal sides, then find the perimeter. Read the side lengths straight from the shape tool, work each perimeter out in your copy, and we check every answer together before the next shape.

What we learned

• Perimeter is the distance all the way around a shape, found by adding every side.
• When sides are equal we can be smart: pair the equal sides of a rectangle, or multiply one side by the number of sides in a regular shape.
• Checking a perimeter two different ways tells us our answer is right.

Coming up

Next we move from the distance around a shape to the flat space inside it. We will start measuring area by counting the square units that cover a shape.