For a rectangle with length l and width w: Perimeter = 2l + 2w. Area = l × w.
For a square with side s: Perimeter = 4s. Area = s².
If you know the area and one side, you can find the other: length = area ÷ width.
Remember
The perimeter formula double-counts both dimensions because rectangles have two of each side: P = 2(l + w).
Real World
Buying carpet for a rectangular room — you need area (l × w) to know how many square metres to order. Buying skirting board — you need perimeter.
Feeling ready?
Put this concept to work with a practice workout.