Metres and centimetres · Usually met around P2
Perimeter: around a shape
Count the outside sides, not the squares.
How to do it
Method
Count the outside sides, not the squares
Length 4, width 2: 4 + 2 + 4 + 2 = 12 cm
Another way
Length plus width, twice
2 × (4 + 2) = 12
In full
The perimeter is the distance all the way round a shape. On squared paper each small square's side is 1 cm: start at a corner and count the squares' sides along the edge until you are back where you started. Count sides, not squares: a corner square has two sides on the outside, so it counts twice. For a rectangle count one length and one width, then the same again: length 4, width 2, 4 + 2 + 4 + 2 = 12. Or think of it as 2 lots of (length + width). Shapes with the same number of squares can have different perimeters.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
Each small square's side is 1 cm. How far is it all the way round the edge of this shape?
Show the answer and working
Answer: 12 cm
Start at a corner and count the small sides along the edge back to the start: 12, so 12 cm.
Example 2
Each small square's side is 1 cm. What is the perimeter of this rectangle?
Show the answer and working
Answer: 16 cm
Length 3 cm, width 5 cm; once round the edge: 3 + 5 + 3 + 5 = 16.
Example 3
Each small square's side is 1 cm. The perimeter of this shape is 14 cm.
True or false?
Show the answer and working
Answer: False
Count the small sides round the outside, not the squares: 12, so 12 cm; the shape has 5 squares.
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