Primary 4 · Mathematics
P4 area and perimeter
One habit costs most of the marks on L-shapes: splitting the figure to find the perimeter, the way you would for the area. Here is the difference, and how to check it at home.
- Level
- P4
- Usually taught
- Term 4
- Covers
- 1 concept
What changes at P4
In Primary 4, shapes become composite: an L or a T made by joining rectangles, and your child finds the area and the perimeter of the whole thing. The two use different moves on the same figure, which is where the topic gets its marks.
Area decomposes the shape into rectangles and adds them up. Perimeter traces the outside boundary once. Reusing the area move for the perimeter is the mistake, because the inner join edge belongs to neither side of the outside.
The sentence that costs the mark
An L-shaped figure has all six of its sides marked. What is its perimeter, the distance all the way round?
What the working usually does
splitsthe L-shape=52 cm
Splits it into two rectangles and adds both perimeters. That is exactly right for the AREA, so it feels right here, but it counts the join edge twice.
What the question asked
tracesthe L-shape=42 cm
Traces the outside boundary once, adding only the outer sides. The inner edge where you would cut is not part of the outside at all.
Splitting into rectangles is the correct move for the area, so a child reuses it here and both totals look tidy. Only running a finger round the outside shows the join edge should never have been counted.
The same question, worked properly
Trace the outside, not the pieces
perimeter = the outer boundaryFor perimeter, run a finger right round the outside of the whole shape. The rectangles you might cut it into are for working out the area, not this.
The inner cut edge does not count
the join edge adds 0This is the step that fails. The dashed line where you would split the shape sits inside it, so it is never part of the outside boundary and adds nothing to the perimeter. Counting it is where the mark goes.
Add the outer sides once
5 + 7 + 5 + 4 + 10 + 11 = 42 cmAdd the six outside lengths, each counted exactly once, working out any unmarked side from the others. The two inner edges of the split are nowhere in this sum.
How can you tell it has clicked?
Ask your child to trace the perimeter with a finger while you watch. If the finger stays on the outside the whole way and never cuts across the middle, they have it. If it crosses the inside, that is the slip.
A quick question also works: "does the perimeter go along the dashed line?" The answer is no, and a child who can say why, because it is inside, not around, has understood it.
One thing to try tonight
Walk the outline of a room that has an alcove, keeping one hand on the wall the whole way. Your hand never crosses the middle of the room. That path is the perimeter. Splitting the room into rectangles is for working out the floor space, the area, a different job.
Then draw an L on paper and have them find both: the area by splitting and adding, the perimeter by tracing round once. Doing them side by side makes the difference stick.
What your child will meet in this topic
- Composite Figures
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
Composite area and perimeter run straight into P5 and P6, where figures get cut from circles and triangles. The same rule holds: perimeter traces the outside, area builds from pieces, and mixing them up is the recurring trap.
A child who keeps the two moves separate here tends to stay steady later, when the shapes are harder and a wrong perimeter quietly loses a whole question.
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 4
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.