The idea in brief. Perimeter is the total length of the boundary, including every inward edge. Trace the outside edge only. Internal joins and shaded squares are not part of the perimeter. In the lesson. Last lesson, you found the area of a figure by counting the square units it covers. Today, we will find the perimeter of a figure by going all the way round it. Perimeter is the total length all the way round a figure. Add every side, even the sides that go in.
Lesson video, with the working
Perimeter
3 min 59 s
The film teaches 3 questions in turn. Each question shows first; its working is written as the film explains it.
Perimeter of a rectangle = length + breadth + length + breadth
Perimeter = 11 + 4 + 11 + 4 = 30 cm
The perimeter of the mat is 30 cm.
Answer: 30 cm
For parents
How do you find the perimeter of rectilinear figures in P3?
A rectilinear figure is a flat shape whose sides are all straight lines meeting at right angles, like an L or a staircase.
What your child learns here
A rectilinear figure is a flat shape whose sides are all straight lines meeting at right angles, like an L or a staircase. This page teaches the meaning of perimeter, the distance all the way round, and how to add it up without leaving a side out.
Perimeter is the total length of the boundary of a plane figure.
Perimeter is a length, so it is measured in cm or m, not in square units.
Every edge on the outside counts, including edges that turn inwards.
A side with no label can often be found from the sides opposite it.
Does the answer make sense?
The perimeter of a figure with only right-angled corners can never be shorter than the distance round the smallest rectangle that fits around it. Your child can add that box's length and breadth and double it; the answer should be that much or more, in cm or m, never in square units.
How to practise these questions
The Basic questions practise adding all the sides of rectangles and squares. The Intermediate questions add rectilinear figures, where some side lengths are not marked and must be found. The Challenge questions ask your child to follow the boundary of a figure with notches or steps, counting each edge only once. If a perimeter question stalls, play its solution film and return to it later from Revision.
Questions parents ask
What is a rectilinear figure? It is a flat shape whose sides are all straight, and where every corner is a right angle, such as an L shape or a staircase shape. The Primary 3 syllabus names the perimeter of rectilinear figures, rectangles and squares.
Is perimeter measured in cm or cm²? Perimeter is a length, so it is written in cm or m. Square units such as cm² are only for area.
How can I help my child remember the difference between area and perimeter? Link perimeter to walking round the edge of a field and area to the grass inside it. On paper, perimeter is traced with a pencil along the outline, while area is shaded in.
