Perimeter of a Rectangle — Formula & Examples

To get the perimeter of a rectangle, add the length and width, then double the result:

Here $l$ is the length and $w$ is the width. Perimeter is the total distance around the outside, so the answer is written in linear units such as cm, m, or ft. The same rule can be written $P = 2l + 2w$ — both forms are correct, and both assume the two sides are in the same unit.

Why Doubling the Sum Works

A rectangle has two sides of length $l$ and two of width $w$. Add all four:

Combine like terms:

Factor out the $2$:

That is the whole reason for the formula. You are measuring a boundary, not the space inside, which is why the result is a length rather than an area.

Worked Example: Length 8 cm, Width 5 cm

A rectangle has length $8$ cm and width $5$ cm. Apply the formula:

Add inside the parentheses, then multiply:

So the perimeter is $26$ cm. Verify by adding all four sides directly:

The results match, so the setup is correct.

Practice and Self-Check

Work this one out: a rectangle with length $12$ m and width $4$ m. Start from $P = 2(l + w)$, then run two checks of your own:

• Is your final unit meters, not square meters?
• Is your answer larger than either single side? It must be, because the perimeter includes all four sides. If it comes out smaller than the length or the width, the setup is wrong.

Common Mistakes

1. Mixing up perimeter and area. Perimeter is the distance around; area is the space inside.
2. Forgetting that each side length appears twice. Using $l + w$ alone misses half the boundary.
3. Mixing units — meters for one side and centimeters for the other without converting first.
4. Writing square units. Perimeter is measured in linear units like cm, m, or ft, never $\text{cm}^2$.

When You Use It

Reach for rectangle perimeter whenever you need the length around a rectangular object: fencing a garden, framing a picture, trimming a window, or measuring the border of a tabletop. If the shape is actually a square, this formula still works because $l = w$, and it simplifies to $P = 4s$.