How to Find the Width of a Rectangle

Co-authored by wikiHow Staff

Last Updated: September 14, 2022 References

There are numerous ways to find a missing dimension of a rectangle, and the method you use will depend on what information you already have. As long as you know the area or perimeter, as well as the length of one side of the rectangle (or the relationship between the length and width), you can find a missing dimension. The properties of a rectangle are such that you can use these methods for finding the width or length.

Method 1

Method 1 of 4:

Using Area and Length

1
Set up the formula for area of a rectangle. The formula is $A=(l)(w)$ $A=(l)(w)$ , where $A$ $A$ equals the area of the rectangle, $l$ $l$ equals the length of the rectangle, and $w$ $w$ equals the width of the rectangle.^{[1]
X
Research source}
- This method will only work if you are given the area and length of the rectangle.
- You might also see the formula written as $A=(h)(w)$ , where $h$ equals the height of the rectangle and is used instead of length.^{[2]
  X
  Research source} These two terms refer to the same measurement.
2
Plug the values for area and length into the formula. Make sure you substitute for the correct variables.
- For example, if you are trying to find the width of a rectangle that has an area of 24 square centimeters, and a length of 8 centimeters, your formula will look like this:
  $24=8w$
Advertisement
3
Solve for $w$ . To do this, you need to divide each side of the equation by the length.^{[3]
X
Research source}
- For example, in the equation $24=8w$ , you would divide each side by 8.
  $24=8w$
  ${\frac {24}{8}}={\frac {8w}{8}}$
  $3=w$
4
Write your final answer. Don’t forget to include the unit of measurement.
- For example, for a rectangle with an area of $24cm^{2}$ and a length of $8cm$ , the width would be $3cm$ .

Method 2

Method 2 of 4:

Using Perimeter and Length

1
Set up the formula for perimeter of a rectangle. The formula is $P=2l+2w$ $P=2l+2w$ , where $P$ $P$ equals the perimeter of the rectangle, $l$ $l$ equals the length of the rectangle, and $w$ $w$ equals the width of the rectangle.^{[4]
X
Research source}
- This method will only work if you are given the perimeter and length of the rectangle.
- You might also see the formula written as $P=2(w+h)$ , where $h$ equals the height of the rectangle and is used instead of length.^{[5]
  X
  Research source} The variables $l$ and $h$ refer to the same measurement, and the distributive property dictates that these two formulas, although arranged differently, will give you the same result.
2
Plug the values for perimeter and length into the formula. Make sure you substitute for the correct variables.^{[6]
X
Research source}
- For example, if you are trying to find the width of a rectangle that has a perimeter of 22 centimeters, and a length of 8 centimeters, your formula will look like this:
  $22=2(8)+2w$
  $22=16+2w$
3
Solve for $w$ . To do this, you need to subtract the length from each side of the equation, then divide by 2.
- For example, in the equation $22=16+2w$ , you would subtract 16 from each side, then divide by 2.
  $22=16+2w$
  $6=2w$
  ${\frac {6}{2}}={\frac {2w}{2}}$
  $3=w$
4
Write your final answer. Don’t forget to include the unit of measurement.
- For example, for a rectangle with a perimeter of $22cm$ and a length of $8cm$ , the width would be $3cm$ .

Method 3

Method 3 of 4:

Using the Diagonal and Length

1
Set up the formula for the diagonal of a rectangle. The formula is $D={\sqrt {w^{2}+l^{2}}}$ $D={\sqrt {w^{{2}}+l^{{2}}}}$ , where $D$ $D$ equals the length of the rectangle’s diagonal, $l$ $l$ equals the length of the rectangle, and $w$ $w$ equals the width of the rectangle.^{[7]
X
Research source}
- This method will only work if you are given the length of the diagonal and the length of the side of the rectangle.
- You might also see the formula written as $D={\sqrt {w^{2}+h^{2}}}$ , where $h$ equals the height of the rectangle and is used instead of length.^{[8]
  X
  Research source} The variables $l$ and $h$ refer to the same measurement.
2
Plug the values for the diagonal and side length into the formula. Make sure you substitute for the correct variables.
- For example, if you are trying to find the width of a rectangle that has a diagonal length of 5 centimeters, and a side length of 4 centimeters, your formula will look like this: $5={\sqrt {w^{2}+4^{2}}}$
3
Square both sides of the formula. You need to do this to get rid of the square root sign, which makes isolating the width variable easier.^{[9]
X
Research source}
- For example:
  $5={\sqrt {w^{2}+4^{2}}}$
  $5^{2}=w^{2}+4^{2}$
  $25=w^{2}+16$
4
Isolate the $w$ variable. To do this, you need to subtract the squared length from each side of the equation.
- For example, in the equation $25=16+w^{2}$ , you would subtract 16 from each side.
  $25=16+w^{2}$
  $9=w^{2}$
5
Solve for $w$ . To do this, you need to find the square root of each side of the equation.
- For example:
  ${\sqrt {9}}={\sqrt {w^{2}}}$
  $3=w$
6
Write your final answer. Don’t forget to include the unit of measurement.
- For example, for a rectangle with a diagonal length of $5cm$ and a side length of $4cm$ , the width would be $3cm$ .

Method 4

Method 4 of 4:

Using Area or Perimeter and Relative Length

1
Set up the formula for area or perimeter of a rectangle. Which formula you use will depend on which measurement you are given. If you are given the area, set up the area formula. If you are given the perimeter, set up the perimeter formula.
- If you do not know the area or the perimeter, or the relationship between the length and the width, you cannot use this method.
- The formula for area is $A=(l)(w)$ .^{[10]
  X
  Research source}
- The formula for perimeter is $P=2l+2w$ .^{[11]
  X
  Research source}
- For example, you might know that the area of a rectangle is 24 square centimeters, so you would set up the formula for area of a rectangle.
2
Write the expression that describes the relationship between the length and the width. Write your expression in terms of what $l$ $l$ equals.
- The relationship might be given by stating how many times bigger one side is than the other, or how many units more or less it is.
- For example, you might know that the length is five centimeters longer than the width. Your expression for the length is then $l=w+5$ .
3
Replace the $l$ variable in your area (or perimeter) formula with the expression for length. Your formula should now only contain the variable $w$ $w$ , which means you can solve for the width.
- For example, if you know that area is 24 square centimeters, and that $l=w+5$ , your formula will look like this:
  $A=(l)(w)$
  $24=(w+5)(w)$
4
Simplify the equation. Your simplified equation might take on various forms, depending on the relationship between the length and the width, and depending on whether you are working with area or perimeter. Think about setting up an equation that allows you to solve for $w$ $w$ in the simplest way.
- For example, you can simplify $24=(w+5)(w)$ to $0=w^{2}+5w-24$ .
5
Solve for $w$ . Again, how you solve for $w$ $w$ will depend on your simplified equation. Use the basic rules of algebra and geometry to solve.^{[12]
X
Research source}
- You might need to use addition or division to solve, or you might need to factor a quadratic equation or use the quadratic formula to solve.
- For example, $0=w^{2}+5w-24$ can be factored as follows:
  $0=w^{2}+5w-24$
  $0=(w+8)(w-3)$
  You will then have two possible solutions for $w$ : $w=3$ or $w=-8$ . Since a rectangle cannot have a negative width, you can eliminate -8. So your solution is $w=3$ .

Community Q&A

Question

How would I find width when I've been given volume, height, and length?

Blankless X

Community Answer

You divide the volume by the height and the length. You could multiply the height and the length with each other first before dividing. W = V / (H*L).
Question

The length of a rectangle field is twice its breadth. Its perimeter is 400m. How do I find the length and breadth?

Donagan

Top Answerer

Let B equal the breadth. Then 2B equals the length. Write an equation in B, stating that the perimeter is equal to twice the breadth plus twice the length. Solve for B.
Question

How do I find the area of a rectangle if width and perimeter are given?

Donagan

Top Answerer

Double the width, and subtract that from the perimeter. Then divide that number by two, which gives you the length of the rectangle. Then, multiply the result by the width to get the area.