Area and Perimeter
Area and perimeter of squares and rectangles, the area of a triangle, composite figures, and finding a side from the area.
⏱ 7 min · 🎯 4 things to master
Perimeter is the fence you walk around a shape; area is the carpet that covers it inside. Mixing these two up is the single most common slip in this whole topic, so we will keep them clearly apart. Master the few formulas here and you can find the size of almost any flat shape on the PSLE — even funny composite ones.
Parents: ask your child to predict whether a question wants area or perimeter before they reveal. The "Method tip" boxes name the exact formula and units a PSLE marker rewards.
By the end you'll be able to find perimeter and area of squares and rectangles, the area of a triangle, the area of composite figures, and a missing side from the area. Let's draw it out.
Perimeter: the distance around
The is the total length of all the sides added together. For a rectangle that is length + width + length + width, which is the same as 2 × (length + width). For a square, all four sides are equal, so the perimeter is 4 × side.
For example, a rectangle 8 cm by 3 cm has perimeter 2 × (8 + 3) = 2 × 11 = 22 cm. A square of side 5 cm has perimeter 4 × 5 = 20 cm. Perimeter is a length, so the unit is plain cm or m.
🤔 Predict first: What is the perimeter of a rectangle 10 cm long and 4 cm wide?
Area of squares and rectangles
The is the space inside the shape. For a rectangle it is length × width, and for a square it is side × side.
For example, a rectangle 8 cm by 3 cm has area 8 × 3 = 24 square cm. A square of side 5 cm has area 5 × 5 = 25 square cm. Because you multiply two lengths, the unit is always square cm or square m.
🤔 Predict first: What is the area of a rectangle 7 cm long and 5 cm wide?
Drag the sides below and watch area and perimeter change — notice they are two different measures that grow at different rates.
Area vs perimeter explorer
Predict first: If you DOUBLE the length, which grows faster?
Area of a triangle
A triangle's area is half × base × height. The is any side you choose, and the is the perpendicular distance from that base to the opposite corner. The height must meet the base at a right angle.
For example, a triangle with base 10 cm and height 6 cm has area half × 10 × 6 = half × 60 = 30 square cm. The height is not a slanted side — it is the straight-up distance, marked with a small square at the base.
🤔 Predict first: A triangle has a base of 8 cm and a height of 5 cm. What is its area?
Composite figures and missing sides
A is built from simpler shapes. To find its area, split it into rectangles and triangles, find each area, then add them up (or subtract a cut-out part).
You can also work backwards. If you know the area of a rectangle and one side, divide to find the other side: a rectangle of area 24 square cm with a width of 4 cm has length 24 ÷ 4 = 6 cm. That is just the area formula rearranged.
🤔 Predict first: A rectangle has an area of 36 square cm and a length of 9 cm. What is its width?
Watch out — these are easily mixed up
Quick recap
🎯 Mastery check
Answer all 6 — your progress is saved on this device.
What is the perimeter of a square with side 9 cm?
What is the area of a rectangle 12 cm long and 5 cm wide?
A triangle has a base of 14 cm and a height of 6 cm. What is its area?
A rectangle has an area of 48 square cm and a width of 6 cm. What is its length?
A square has an area of 49 square cm. What is the length of one side?
A figure is a 10 cm by 4 cm rectangle with a triangle of base 4 cm and height 3 cm joined on top. What is the total area?