Primary 3 · Mathematics

P3 area and perimeter

Area and perimeter use the same two measurements, so it is easy to run the wrong method and still land a tidy number. Most marks here are lost by adding the sides when the question wants the space inside. Here is how to tell the two apart.

Level
P3
Usually taught
Term 4
Covers
3 concepts

What changes at P3

In Primary 3, your child measures two different things about a flat shape: the perimeter, the distance all the way around the edge, and the area, the amount of space inside. Both come from the same side lengths, which is exactly why they get mixed up.

The one thing to get right is which question is being asked. Perimeter is a length, measured in cm. Area is a covering, measured in square centimetres, cm². Knowing which one the words want is the whole skill at this level.

The sentence that costs the mark

A rectangle has sides of 5 cm and 3 cm. What is its area?

What the working usually does

addthe sides=16 cm

Adds all four sides. That gives 16, but 16 cm is the perimeter, the distance around, not the area.

What the question asked

multiplythe sides=15 cm²

Multiplies length by width to cover the inside. Area is counted in squares, so the unit is cm².

Both use the same two numbers and both give a real answer, so nothing looks careless. The unit is the giveaway: a distance around is in cm, a space inside is in cm². Ask which the question wants, the edge or the inside, before picking a method.

5 cm3 cm5 cm3 cm
Perimeter: go around the edge and add the four sides.
5 cm3 cm
Area: cover the inside with unit squares, then count them.

The same question, worked properly

  1. Ask which one the question wants

    space inside, not distance around

    The words decide. "Area" means the space inside, so it is length times width. "Perimeter" means the distance around, so it is the four sides added. Read the question before reaching for a method.

  2. Area multiplies, it does not add the sides

    5 × 3, not 5 + 3 + 5 + 3

    This is the step that fails. Adding the four sides gives 16, but that is the perimeter. Area covers the inside, so multiply the length by the width. This is exactly where the method gets swapped and the mark is lost.

  3. Write it in square units

    5 × 3 = 15 cm²

    Area is counted in squares, so the unit is cm², not cm. That small raised 2 is the sign that the answer measures a space, not a line.

How can you tell it has clicked?

Draw a rectangle and ask two things about it: how far it is around the edge, and how much space is inside. A child who reaches for adding on the first and multiplying on the second has understood the difference. One who uses the same method for both has not.

The quickest tell is the unit. If the answer to an area question comes back in cm rather than cm², the method used was the perimeter one, whatever number sits in front.

One thing to try tonight

Use a tile or a place mat. Run a finger all the way around the edge, that is the perimeter, a length. Then press a flat palm on the top, that is the area, a covering. Same rectangle, two different questions.

When a question says "area", ask your child to say "space inside" out loud before working; for "perimeter", "distance around". Naming the job first stops the method swap before it starts.

What your child will meet in this topic

  • Rectangle Perimeter
  • Rectangle Area
  • Square Area

Most Singapore schools introduce this in Term 4, though schools do differ by a term either way. If your child’s school runs a different order, you can set it in the schedule on your dashboard.

What it leads to

This split runs straight into P4, where the shapes become L-shapes and composite figures but the two jobs stay the same: trace the boundary for perimeter, cover the inside for area. A child solid on the P3 rectangle meets those without relearning anything.

Area in square units is also the groundwork for volume in cubic units later. The idea that the unit tells you what is being measured, a line, a surface, or a solid, starts right here.

The same topic in other years

It comes back each year and gets harder, so it helps to see what came before and what is coming.

More in Primary 3

Other topics in the same year, so you can see what sits either side of this one.

Struggling with the working rather than the topic itself? How to help your child with problem sums

See how your child is doing on this topic

StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.

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