Rectangle Perimeter – Formula & Examples

Guide / Article

How to Find the Perimeter of a Rectangle – Formula & Examples

Guide / Article·Last updated: September 19, 2026·Sources·Methodology

Rectangle geometry guide

How to Find the Perimeter of a Rectangle – Formula, Reverse Problems & Examples

The rectangle perimeter formula comes directly from the fact that opposite sides are equal. It can also be rearranged to find a missing length or width.

Quick answerUse P=2(L+W). Add length and width, then double the result. If perimeter and one dimension are known, the missing dimension is P/2 minus the known dimension.

On this page

Formula
Step-by-step example
Reverse problems
Square as a special case
Worked examples
Common mistakes
Practice

Rectangle Perimeter Formula

P = 2L + 2W = 2(L+W)

A rectangle has two equal lengths and two equal widths, so the formula is simply a compressed form of L+W+L+W.

Step-by-Step Example

Rectangle: L=9 m, W=4 m.

P=2(9+4)=2×13=26 m

Direct addition gives the same answer: 9+4+9+4=26.

Reverse Problems — Find a Missing Length or Width

Start with P=2(L+W). Divide by 2:

P/2 = L+W

Then:

L=P/2−W or W=P/2−L

Example: P=50 m and W=9 m:

L=25−9=16 m

Rectangle vs Square

A square is a special rectangle with L=W=s. Substituting into P=2(L+W):

P=2(s+s)=4s

This shows why the square perimeter formula is not separate mathematics—it is a special case of the rectangle formula.

Worked Examples

12 ft by 7 ft rectangle

P=2(12+7)=38 ft

Perimeter 64 cm, length 20 cm — find width

W=64/2−20=32−20=12 cm

Fence around 25 m by 18 m plot

P=2(25+18)=86 m

This is geometric boundary length before gates, overlaps or project allowances.

Common Mistakes

Using L×WThat calculates area, not perimeter.

Doubling only one dimensionBoth length and width occur twice.

Square unitsPerimeter uses m, ft, cm—not m² or ft².

Mixed unitsConvert before adding.

Practice

L=15, W=6

Answer: P=2(21)=42.

P=70, L=22 — find W

Answer: W=35−22=13.

Questions People Ask

Open any question for a detailed explanation, not just a one-line answer.

What is the perimeter formula for a rectangle?

P=2L+2W, usually written as P=2(L+W). It represents the two equal lengths plus the two equal widths.

Why do you multiply by 2?

Because a rectangle has two sides of length L and two sides of width W.

Can I find a missing side from perimeter?

Yes. Divide the perimeter by 2, then subtract the known other dimension: L=P/2−W or W=P/2−L.

Is rectangle perimeter the same as area?

No. Perimeter is distance around the boundary and uses linear units. Area is L×W and uses square units.

How do I calculate fencing for a rectangular plot?

Geometrically, start with P=2(L+W). Real fencing may then require adjustments for gates, overlaps, corners or site conditions.

What if length and width use different units?

Convert both to the same length unit before adding. For example, convert centimetres to metres or feet/inches consistently.

What happens if the rectangle is actually a square?

Then L=W=s and the formula simplifies to P=4s.

Can I use the Perimeter Calculator for reverse problems?

Use the Perimeter Calculator for common shape boundary calculations; this guide shows the algebra for reverse rectangle problems.

Related CalcTypes Resources

Methodology

The formula is derived from repeated addition of opposite sides. Reverse examples use direct algebraic rearrangement without changing the underlying geometry.

Sources