Thursday, 23 August 2012

Calculating the Area of a Rectangle

...and how it relates to multiplication facts

Alright - we all know this remarkable feature of multiplication.

And we already know the formula because someone taught us back in high school: 
area = base x height

But what about my team of Year 4 kids? 
What links could they find between area and multiplication facts?

So we started with some coloured rectangles, carefully cut to be pretty close to whole centimetres.

Then, using our multilink cubes we worked out how many we would need to cover the shape.

Much discussion ensued using words such as "area", "length", "height", "centimetre", "compare", "larger", "smaller" etc

"Ah, yes! 3 rows of 6 cubes. So the area is 18 square centimetres!"

And then the moment of epiphany!
Like a crack in the dam wall, the realisation came that we don't need to cover the whole shape.
We can in fact use our knowledge of multiplication facts to simplify this task.

So instead of putting out 150 centicubes, I only need to know the length of the base and the height.

Viola! 10 x 15 = 150

The final step was to compare the areas of larger shapes.
Who had the biggest rectangle? How do we know? How can you prove it?

So what?

This may not seem too spectacular for a lesson idea, but the lights really came on for a few individuals in the class. Sometimes what seems obvious to us adults really is remarkable to kids.


  1. Seriously a great idea! I think that when I get to teaching area and perimeter with my resource classes I will definitely use an activity like this to help them visualize and manipulate things. Thanks for the inspiration!

  2. The lesson idea is a good one and one I had used many times. Merely giving abstract equations for calculating area may mean children will calculate correctly but this hands-on experience will lead to them understanding why the area calculation works.


  3. Hi Bruce,

    Congratulations on the fellowship! Love the blog and have pinched some of your ideas to use. Will catch up on twitter again soon.

    Best wishes
    Jamie Richards


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