Perimeter of a Rectangle — Formula & Examples

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

$$P = 2(l + w)$$

Here, $l$ is the length and $w$ is the width. Perimeter means the total distance around the outside of the rectangle, so the answer should be written in linear units such as cm, m, or ft.

You can also write the same formula as $P = 2l + 2w$. Both forms are correct. This only works if both side lengths are in the same unit.

Rectangle Perimeter Formula Explained

A rectangle has two sides of length $l$ and two sides of width $w$. If you add all four sides, you get

$$l + w + l + w$$

Combine like terms:

$$2l + 2w$$

Then factor out $2$:

$$2(l + w)$$

That is why the perimeter formula works. Perimeter measures a boundary, not the space inside the shape.

Example: Length 8 cm, Width 5 cm

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

$$P = 2(l + w)$$

Substitute the values:

$$P = 2(8 + 5)$$

Add inside the parentheses and multiply:

$$P = 2(13) = 26$$

So the perimeter is $26$ cm. A quick check is to add all four sides directly:

$$8 + 5 + 8 + 5 = 26$$

The result matches, so the setup is correct.

Common Mistakes With Rectangle Perimeter

1. Mixing up perimeter and area. Perimeter is distance around the shape, while area is the space inside it.
2. Forgetting that each side length appears twice. Using $l + w$ alone does not give the full boundary.
3. Mixing units, such as using 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, not $\text{cm}^2$.

When You Use The Perimeter Of A Rectangle

Use rectangle perimeter when you need the length around a rectangular object. Common cases include fencing a garden, framing a picture, adding trim around a window, or measuring the border of a tabletop.

If the shape is actually a square, the rectangle formula still works because $l = w$. In that case, it simplifies to $P = 4s$.

Quick Check Before You Move On

Your perimeter should be larger than either single side, because it includes all four sides. If your answer is smaller than the length or the width, something is wrong in the setup.

Try A Similar Problem

Try your own version with length $12$ m and width $4$ m. Start with $P = 2(l + w)$, then check whether your final unit is written as meters, not square meters. If you want to explore another geometry case after that, try a similar problem and compare the setup.