To find the perimeter of a rectangle, add all four sides or use the formula P=2(l+w)P=2(l+w)P=2(l+w), where lll is length and www is width.

How to Find the Perimeter of a Rectangle

Quick Scoop

If you can measure the **length** and **width** of a rectangle, you already have everything you need to get its perimeter. The perimeter is just the distance all the way around the shape, like walking once around a rectangular field or putting a fence around a garden.

What “Perimeter” Really Means

Perimeter = total distance around a shape.

For a rectangle, there are:
- 2 equal lengths
- 2 equal widths

So the perimeter is “length + length + width + width”.

Write that as a formula:

P=l+l+w+wP=l+l+w+wP=l+l+w+w

Combine like terms:

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

Step‑by‑Step: Basic Formula

Here’s the usual way students learn how to find the perimeter of a rectangle.

Measure or read the length $$l$$.
Measure or read the width $$w$$.
Add them: $$l + w$$.
Multiply by 2: $$P = 2(l + w)$$.
Attach the correct units (cm, m, ft, etc.).

Example: A rectangle has length 12 cm and width 8 cm.

P=2(l+w)=2(12+8)=2×20=40textcmP=2(l+w)=2(12+8)=2\times 20=40\\text{cm}P=2(l+w)=2(12+8)=2×20=40textcm

Another example:
Length 15 m, width 9 m.

P=2(l+w)=2(15+9)=2×24=48textmP=2(l+w)=2(15+9)=2\times 24=48\\text{m}P=2(l+w)=2(15+9)=2×24=48textm

Mini Section: Why the Formula Makes Sense

You can imagine a rectangle as a simple walk:

Walk the length.
Turn, walk the width.
Walk the same length again.
Walk the same width again.

So you really walk “length + width + length + width” = 2l+2w2l+2w2l+2w. Factoring out the 2 gives 2(l+w)2(l+w)2(l+w), which is shorter to remember and easier to compute.

Think of it like this: instead of adding four separate sides every time, you add one length and one width, then just double it.

Other Situations You Might See

Sometimes problems give you different information: diagonal, area, or only one side. Here’s how that connects back to perimeter.

Perimeter When You Know the Diagonal

If you know the diagonal ddd and one side (say the length lll), you can use the Pythagorean theorem:

d2=l2+w2⇒w=d2−l2d^2=l^2+w^2\Rightarrow w=\sqrt{d^2-l^2}d2=l2+w2⇒w=d2−l2

Then plug into the perimeter formula:

P=2(l+w)=2(l+d2−l2)P=2(l+w)=2\Bigl(l+\sqrt{d^2-l^2}\Bigr)P=2(l+w)=2(l+d2−l2)

Some references write it in the expanded form P=2l+2d2−l2P=2l+2\sqrt{d^2-l^2}P=2l+2d2−l2.

Perimeter When You Know the Area

If you know the area AAA and one side:

Use A=l×wA=l\times wA=l×w.

So, if you know lll: w=Alw=\dfrac{A}{l}w=lA.

Then perimeter: P=2(l+w)=2(l+Al)P=2(l+w)=2\left(l+\dfrac{A}{l}\right)P=2(l+w)=2(l+lA).

Example idea (no full numbers needed):
A rectangle has area 60 m² and length 12 m.

w=60/12=5w=60/12=5w=60/12=5 m
P=2(12+5)=34P=2(12+5)=34P=2(12+5)=34 m

Forum-Style Quick Q&A

Q: What’s the easiest way to remember how to find the perimeter of a rectangle? A: Add the length and width once, then double the result: $$P = 2(l + w)$$.[3][5][1]

Q: Is perimeter the same as area? A: No. Perimeter is the distance around the rectangle ($$2(l + w)$$), while area is the space inside ($$A = l \times w$$).[7][1]

Q: Do the units change when finding the perimeter? A: No. If your sides are in cm, the perimeter is in cm; in meters, the perimeter is in meters, etc.[10][1]

Mini Section: Real-Life Uses (2020s & Today)

Knowing how to find the perimeter of a rectangle shows up in everyday modern tasks:

Planning a fence around a rectangular yard or garden.
Buying enough LED strip lights to go around a rectangular TV or monitor frame.
Ordering border tiles for a rectangular bathroom or kitchen floor.
Estimating how much trim you need around a rectangular door or window.

In each of these, you measure length and width, calculate 2(l+w)2(l+w)2(l+w), then buy that much material plus a small extra margin.

SEO Mini Section: Key Tips at a Glance

Use the formula: P=2(l+w)P=2(l+w)P=2(l+w).

Make sure length and width use the same units.
For word problems, identify what represents length and what represents width before calculating.
If given diagonal or area, convert that info to find the missing side, then use the same perimeter formula.

Short TL;DR

Perimeter of a rectangle = distance all the way around.

Formula: P=2(l+w)P=2(l+w)P=2(l+w).

If you know area or diagonal, first find the missing side, then still use P=2(l+w)P=2(l+w)P=2(l+w).

Information gathered from public forums or data available on the internet and portrayed here.

how to find the perimeter of a rectangle