Add the outside edges
A shape with sides 8, 3, 5 and 6 has perimeter 22 if those are all the outside lengths.
Perimeter uses the outside only.
Year 5
Measure and calculate the perimeter of composite rectilinear shapes in centimetres and metres
Find the perimeter of composite rectilinear shapes by adding all outer side lengths.
Lesson overview
Measurement
Perimeter is the total distance around the outside of a shape. For composite rectilinear shapes, it helps to break the outline into straight sections and make sure every outside edge is counted once.
Use missing lengths
Some composite shapes need you to work out a hidden side first.
missing part is 4 m
Use matching horizontal or vertical lengths.
Keep units consistent
Do not mix centimetres and metres without converting first.
Use one unit before adding.
Worked example
A composite rectilinear shape has outside lengths 6 m, 4 m, 2 m, 3 m, 4 m and 7 m. What is the perimeter?
- List every outside edge once.
- Add the lengths carefully.
- 6 + 4 + 2 + 3 + 4 + 7 = 26.
- The perimeter is 26 m.
Round 1: Learn it
Count each outside edge once and keep the units consistent.
Finish this round to unlock Round 2: Show you know it.
Question 1
What is the perimeter of sides 8 m, 3 m, 5 m and 6 m?
Question 2
If the whole top is 10 m and one bottom part is 6 m, what is the missing bottom part?
Question 3
What is 2 m in centimetres?
Question 4
Find the perimeter of outside lengths 6, 4, 2, 3, 4, 7 metres.
Question 5
Which line should you ignore when finding perimeter?
outside only
inside lines
Common mistakes
Watch for these as you work through the lesson.
- Missing one outside edge.
- Including an inside line that is not part of the perimeter.
- Adding lengths with mixed units.
Related topics
These ideas connect closely to this lesson.
- Convert between different units of metric measure and understand approximate equivalences between metric units and common imperial units.
- Calculate and compare the area of rectangles, and estimate the area of irregular shapes.