Key takeaways
- The perimeter of a rectangle is P = 2(l + w).
- Perimeter uses regular units (m, ft), never squared units.
- A rectangle has two lengths and two widths, so the sum is doubled.
- The same perimeter can enclose very different amounts of area.
In this guide
What is the perimeter of a rectangle?
Perimeter is the distance all the way around the outside of a shape, measured in regular units like meters or feet, not square units.
Why does the perimeter of a rectangle matter?
Perimeter tells you exactly how much fencing, edging, or trim to buy, regardless of what shape the rectangle is.
A 10 m × 6 m yard and a 12 m × 4 m yard both need exactly 32 m of fencing to enclose them — even though the first yard has 60 m² of space inside and the second only has 48 m².
Perimeter and area measure two completely different things, and they don't move together.
The formula
| Symbol | What it means | Example |
|---|---|---|
| P | The perimeter, total distance around | 32 m |
| l | The length: the longer side | 10 m |
| w | The width: the shorter side | 6 m |
A rectangle has two lengths and two widths, which is why the sum (l + w) gets multiplied by 2.
How to find the perimeter step by step
- 1Measure the length. Note the longer side.
- 2Measure the width. Note the shorter side.
- 3Add them together.
l + w. - 4Multiply by 2. This accounts for the second length and second width.
- 5Add regular units. Meters or feet, not squared.
Try it below: change the length and width and watch the fencing total update, with every step shown.
Try it yourself
Pre-filled with the example — change any value.
Common mistakes to avoid
- Only adding two sides. A rectangle has four sides, so you need
2 × (length + width), not justlength + width. - Forgetting the two equal pairs. You never need to measure all four sides individually; opposite sides are always equal.
- Assuming a bigger perimeter means a bigger space inside. As shown above, it doesn't always.
Tips and tricks
- For quick mental math, double the length and width separately, then add:
2l + 2w. - If every side is equal, you're really working with a square, so use
4sinstead.
Where you'll use it in real life
- Landscaping: buying fencing for a rectangular yard.
- Gardening: trimming a border around a rectangular garden bed.
- Crafts: working out ribbon needed to wrap a rectangular box.
- Construction: estimating baseboard or trim for a rectangular room.
Quick summary
To find the perimeter of a rectangle, add the length and width, then double the result. The answer is always in regular units, not squared ones. Use the calculator above to check your fencing or trim total instantly.
Worked example
You want to put a fence around a rectangular yard that is 10 meters long and 6 meters wide. How much fencing do you need to buy?
- 01P = 2(l + w)
- 02P = 2(10 + 6)
- 03P = 2(16)
- 04P = 32 m
These numbers are pre-filled in the calculator above.
Frequently asked questions
How do I find the perimeter of a rectangle with only one side and the area?
Divide the area by the known side to find the other side, then add both sides and multiply by 2.
What's the difference between perimeter and area of a rectangle?
Perimeter (2(l + w)) is the distance around the edge, in units like meters. Area (l × w) is the space inside, in square units like m². Use the [area of a rectangle calculator](/calculator/area-rectangle) for the space inside.
Does a longer rectangle always have a bigger perimeter than a shorter one?
Not necessarily. Two rectangles can have the same perimeter with very different shapes, for example 10 × 6 and 12 × 4 both have a perimeter of 32.
How do I find the perimeter if I only know the diagonal?
The diagonal alone isn't enough, since many rectangles share the same diagonal. You need at least one side too — see the [diagonal of a rectangle calculator](/calculator/diagonal-rectangle) for how the diagonal relates to the sides.
