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📏 Topics & explainers · 5 min read

Perimeter for Class 4: walking around the edge of a shape

What perimeter means, the rectangle and square formulas Class 4 children need, worked examples, and the perimeter-vs-area trap in SOF IMO papers.

Perimeter is the distance all the way around the edge of a shape — imagine an ant walking along the boundary and back to where it started. If a rectangle is 8 cm long and 5 cm wide, the ant walks 8 + 5 + 8 + 5 = 26 cm. Class 4 questions dress this simple idea up in fences, picture frames and bent wires.

The idea in one minute

  • Perimeter of any shape with straight sides: add up all the side lengths.
  • Rectangle shortcut: perimeter = 2 × (length + width), because each pair of opposite sides repeats.
  • Square shortcut: perimeter = 4 × side, and going backwards, side = perimeter ÷ 4.
  • Perimeter is a length, so the answer is in cm or m — never square cm, which belongs to area.

✏️ Warm-up: perimeter of a rectangle

A rectangle is 8 cm long and 5 cm wide. What is its perimeter?

  1. A26 cm
  2. B13 cm
  3. C40 cm
  4. D21 cm
Show the answer

Answer: 26 cm. Perimeter = 2 × (length + width) = 2 × (8 + 5) = 2 × 13 = 26 cm. The traps: 13 cm is only half the walk (one length plus one width), and 40 cm is 8 × 5 — the area, not the perimeter.

✏️ Level up: work backwards from the perimeter

The perimeter of a square is 36 cm. How long is each side?

  1. A6 cm
  2. B9 cm
  3. C12 cm
  4. D18 cm
Show the answer

Answer: 9 cm. A square has 4 equal sides, so one side is the perimeter divided by 4: 36 ÷ 4 = 9 cm. Check by going forwards: 4 × 9 = 36 cm. A child who divides by 2 gets the trap answer 18, and one who divides by 6 gets 6.

✏️ Olympiad twist: the bent wire

A wire 24 cm long is bent into a rectangle whose length is 8 cm. What is the width of the rectangle?

  1. A4 cm
  2. B8 cm
  3. C16 cm
  4. D3 cm
Show the answer

Answer: 4 cm. The wire’s full length becomes the perimeter, so 2 × (length + width) = 24, which means length + width = 12. With the length 8 cm, the width is 12 − 8 = 4 cm. Check: 8 + 4 + 8 + 4 = 24 cm of wire. The trap answer 16 comes from doing 24 − 8 and forgetting there are two lengths and two widths.

Perimeter has a dedicated run of levels in LittleMathematicians’s Class 4 Geometry topic, moving from straightforward rectangles to bent-wire and missing-side puzzles as your child’s mastery grows. It is free during early access — a nice topic to watch the adaptive levels do their work.

Practice this the fun way

Adaptive levels, exam-pattern mocks and progress you can see — free during early access.

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Keep reading

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