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.