Perimeter and area
Perimeter goes round, area fills in
The perimeter is the distance all the way round the edge, in cm or m.
The area is the space inside, in cm² or m².
rectangle: area = length × width
triangle: area = ½ × base × perpendicular height
parallelogram: area = base × perpendicular height
trapezium: area = ½ × (a + b) × h
In the trapezium, a and b are the two parallel sides and h is the distance between them.
Not drawn accurately
- area = ½ × (4 + 9) × 3
- = ½ × 13 × 3
- = 19.5 cm²
The height must be perpendicular
The height in every one of these formulas meets the base at a right angle.
A sloping side is not the height. In a diagram, look for the dashed line with the small square.
For an L-shape, split it into two rectangles. Work out any missing lengths first: if the whole bottom is 12 cm and the top part is 5 cm, the step is 12 − 5 = 7 cm. The perimeter needs every side, including the ones you worked out.
Area units are squared
1 m = 100 cm, but 1 m² is a square 100 cm by 100 cm, so 1 m² = 10 000 cm².
In the same way 1 cm² = 10 × 10 = 100 mm².
2.5 m² = 2.5 × 10 000 = 25 000 cm², not 250 cm².
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