How do I calculate a rectangle's area?
Multiply length by width. A 8 m by 5 m room is 40 square metres. Unlike a square, the two dimensions differ, so you need both.
// maths › 2D Shapes
Rectangle Area Calculator
Find the area, perimeter and diagonal of a rectangle from its length and width.
area = length×width ; perimeter = 2(length+width) ; diagonal = √(l²+w²)
Frequently asked questions
What is the perimeter?
The distance all the way around: twice the length plus twice the width, or 2(l + w). For 8 by 5 that is 26 units, which is how much fencing or skirting you would need.
How is the diagonal found?
With Pythagoras: the square root of length squared plus width squared. It is the longest straight line across the rectangle, corner to corner.
Can a rectangle be a square?
Yes, a square is just a rectangle whose length and width are equal. The same formulas work; the diagonal then simplifies to side times root 2.
Where would I use this?
Flooring, painting walls, sizing a garden bed, or checking whether a rectangular table will fit diagonally through a hallway.