What is perimeter?
The perimeter of a two-dimensional shape is the total length of its boundary — the distance you would walk if you went all the way around the edge and back to the start. Because it is a length, perimeter is measured in length units such as centimetres (cm), metres (m) or kilometres (km).
For any polygon, you find the perimeter by adding the lengths of all its sides. Some shapes have handy shortcuts:
Key idea / 要点
Square: P = 4 × side. Rectangle: P = 2 × (length + width). Any polygon: P = sum of all sides.
Worked example
Worked example / 例题
A rectangle is 8 cm long and 5 cm wide. Find its perimeter.
P = 2 × (length + width) = 2 × (8 + 5) = 2 × 13 = 26 cm.
You can also work backwards. If a square has a perimeter of 36 cm, then side = 36 ÷ 4 = 9 cm. If a rectangle has perimeter 30 cm and length 10 cm, then length + width = 30 ÷ 2 = 15 cm, so width = 15 − 10 = 5 cm.
Composite shapes
For an L-shape or other combined figure, still travel around the whole outline and add every edge — do not miss the short inside steps. Make sure all lengths use the same unit before adding.
Remember / 记住
- Perimeter is a length, so give a length unit (cm, m).
- Add every side, including hidden or unlabelled ones.
- Change all measurements to the same unit first.