Perimeter of Compound Shapes
Measure and calculate the perimeter of composite rectilinear shapes in centimetres and metres
What a learner can do afterwards
- Find the perimeter of an L-shaped figure by identifying all side lengths
- Calculate the perimeter of a composite shape where some sides must be deduced
- Draw a composite rectilinear shape with a perimeter of 40 cm
The lesson
You already know how to find the perimeter of a shape: add up the length of every side. A compound shape, like an L-shape, is made when two rectangles are joined together. To find its perimeter, add up every outside edge, even the short ones that make the step in the L.
Some sides in a compound shape have no number written on them. You can still work them out, because the short step sides must add up to match the long straight side facing them.
A shape is an L. Going around: bottom = 10 cm, right side (short) = 4 cm, top (short) = 4 cm, left side = 10 cm. Two step sides in the middle have no number. The bottom (10 cm) must match the two top pieces added together, so the missing top piece is 10 - 4 = 6 cm. The left side (10 cm) must match the two right pieces added together, so the missing right piece is also 10 - 4 = 6 cm. Now add all six sides: 10 + 4 + 6 + 6 + 4 + 10 = 40 cm.
Count the sides before you add them up. It is easy to skip a short step side by accident, and every side counts toward the perimeter.
To find the perimeter of a compound shape, add every outside side, working out any missing ones first.
Watch it
Where it sits
This opens up
Nothing builds on it yet.
Where this leads
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.