Area of rectangles

Watch it happen

Build the area, row by row

A rectangle 14 cm long and 12 cm wide, marked out in centimetre squares. Its area is the number of squares that cover it.

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The idea

Area is the amount of surface a shape covers, measured in square units such as cm² or m². For a rectangle, imagine covering it with rows of unit squares: the number of squares in each row is the length, and the number of rows is the width — so area = length × width.

A square is just a rectangle with equal sides, so its area is side × side.

Keep area and perimeter separate: perimeter is the distance around the edge (a length, in cm or m), while area is the covering inside (in square units). If your answer's unit is not squared, you have probably found the perimeter.

Worked example

The rectangle in the pictures is 10 cm long and 11 cm wide. What is its area?

1. Cover the rectangle with centimetre squares. The first row holds 10 squares — one for each centimetre of length.

   

2. There are 11 rows altogether. Counting in rows: 10 × 11 = 110 squares, so the area is 110 cm². (Half-way you had 5 rows filled: 5 × 10 = 50.)

   

3. Do not confuse area with perimeter: the perimeter walks the edge, 10 + 11 + 10 + 11 = 42 cm. Area counts the squares inside and is measured in cm².

   

Answer: 110 cm² — area = length × width = 10 × 11 = 110 cm².

Try one yourself

A rectangle is 3 cm long and 10 cm wide. What is its area?

Watch out

• Adding the length and width (or doubling and adding for perimeter) when the question asks for area.
• Forgetting the squared unit — writing 36 cm instead of 36 cm².
• Counting squares around the edge instead of covering the whole surface.