The area of a rectangle, a square and a triangle

What is area?

Area measures the surface occupied by a shape, that is, the amount of ‘paint’ needed to cover it entirely. It is expressed in square units: cm², m², km²...

Area of a rectangle and a square

For a rectangle with length L and width l: A = L × l

For a square with side length c: A = c² (i.e. c × c)

Area of a triangle

A = (base × height) / 2

Note: the height must always be perpendicular (form a right angle) to the chosen base; it is not necessarily one of the triangle’s sides.

Example

Figure	Dimensions	Calculation	Area
Rectangle	L=9 cm, l=4 cm	9 × 4	36 cm²
Square	c=6 cm	6²	36 cm²
Triangle	base=8 cm, height=5 cm	(8×5)/2	20 cm²

Pitfalls to avoid

Do not confuse L × l (area) with 2 × (L + l) (perimeter): these are two different calculations and two different units. For the triangle, make sure the height corresponds to the base used and don’t forget to divide by 2, otherwise the result will be double the expected value.