Calculate perimeter - Rectangles & squares
Theory
The perimeter is the total distance around the outside of a shape. For a rectangle, add all four sides or use \(P=2(l+w)\); for a square, use \(P=4s\).
The perimeter is the distance right around the edge of a flat shape.
A rectangle has two lengths and two widths, so \(P=l+w+l+w=2(l+w)\).
A square has four equal sides, so \(P=4s\). To work backwards, \(s=P\div 4\).
These formulas give the perimeter both ways.
| Shape | Perimeter | Backwards |
|---|---|---|
| Rectangle | \(P=2(l+w)\) | \(w=(P\div 2)-l\) |
| Square | \(P=4s\) | \(s=P\div 4\) |
How to find a perimeter
- Rectangle: add a length and a width, then double.
- Square: multiply one side by \(4\).
- Backwards: divide or subtract to find a missing side.
| \(P\) | \(=\) | \(8+5+8+5=26\) |
The perimeter is \(26\) cm.
| \(P\) | \(=\) | \(4\times 9=36\) |
The perimeter is \(36\) cm.
| \(P\) | \(=\) | \(2\times(15+4)=38\) |
The perimeter is \(38\) cm.
| \(s\) | \(=\) | \(48\div 4=12\) |
Each side is \(12\) cm.
Common pitfalls
Frequently asked questions
How do you find the perimeter of a rectangle?
Add all four sides, or add one length and one width and double the total: \(P=2(l+w)\). For \(8\) cm by \(5\) cm, \(P=26\) cm.
How do you find the perimeter of a square?
Multiply one side by \(4\), because all four sides are equal. A square with \(9\) cm sides has \(P=4\times 9=36\) cm.
How do you find a side from the perimeter?
For a square, divide the perimeter by \(4\). For a rectangle, halve the perimeter and take the known side away.
What is the difference between perimeter and area?
Perimeter is the distance around the edge, measured in cm or m. Area is the space inside, measured in square units.