Perimeter

Objective. Calculate and solve problems involving the boundary of shapes.

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2. Worked example
3. Practice
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Learn: the key idea

The perimeter of a shape is the total distance measured around its outer boundary, found by adding together the lengths of all its sides. For a rectangle, this can be calculated using the formula P=2l+2w, where l is the length and w is the width, since opposite sides are equal. For an irregular shape, every side length must be added individually, including any sides that are not directly labelled but can be worked out from the given measurements. Always check the units of each side length match before adding them together.

Worked example

Find the perimeter of a rectangle with length 9 cm and width 5 cm.

1 $Recall the formula for perimeter: P=2l+2w.$
2 $Substitute the values: P=2\times9+2\times5.$
3 $Calculate each part: 2\times9=18 and 2\times5=10.$
4 $Add the results: 18+10=28.$

Answer: 28 cm

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Common mistake. Pupils sometimes forget to double both the length and width of a rectangle, adding them only once instead of accounting for both pairs of equal sides. Always count every side around the shape, not just the two given lengths.

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