How to Find the Area of a Rectangle Using the Diagonal

Co-authored by David Jia and Hannah Madden

Last Updated: March 31, 2023 References

When you’re working with rectangles, you can find out a lot of information about them just by knowing a few key points of information. If you’ve been given the length of the diagonal and at least one side, you can calculate the area of the whole shape with just a few equations. Always use a calculator to check your math, and don’t forget to add your units onto the answer!

Method 1

Method 1 of 2:

Pythagorean Theorem

1
Mark the diagonal on the rectangle. If you have a visual of your rectangle, it will be a lot easier to figure out the area given a diagonal. Draw out a rectangle (it doesn’t have to match the proportions given, just a rectangle shape will do) and mark out a diagonal line to create 2 triangles.^{[1]
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- You’ll be able to see the 2 triangles that make up the rectangle now.
2
Assign 2 sides of the triangle as $a$ and $b$ . Pick one of the triangles that you’ve created in your rectangle to work with. Assign one side as $a$ $a$ and the other side as $b$ $b$ . Make sure that you know either the length or width value as well as the diagonal value to solve your equation.^{[2]
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- If you were working with a square, you could assign both sides of the triangle as $a$ since they are both the same.
- You can use the Pythagorean Theorem because you’re working with a right angle triangle.
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3
Plug the 2 measurements into the Pythagorean Theorem. The Pythagorean Theorem states that $a^{2}+b^{2}=c^{2}$ $a^{2}+b^{2}=c^{2}$ . In this instance, the height and width of the triangle are $a$ $a$ and $b$ $b$ , while $c$ $c$ is the diagonal.^{[3]
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- Let’s say the diagonal is 10 inches and the length is 8 inches. The equation would look like this: $a^{2}+8^{2}=10^{2}$ .
4
Solve the equation to find the missing side. Break down every number to its simplest form one step at a time to solve for the missing expression. When you’re done, you can assign the answer to the missing side on your rectangle.^{[4]
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- For example: $a^{2}+8^{2}=10^{2}$ .
- $a^{2}+64=100$ .
- $a^{2}=100-64$ .
- $a^{2}=36$ .
- ${\sqrt {a^{2}}}={\sqrt {36}}$ .
- $a=6$ .
5
Plug the new number into $A=l\times w$ . To find the area of your entire rectangle, you need to solve for Area = length x width, or A = l x w. Now that you’ve used the Pythagorean Theorem to find the second side length, you can solve for the area. Don’t forget your units!^{[5]
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- For example: $A=8\times 6$ .
- $A=48\ in^{2}$ .
- Area is always in units squared.

Method 2

Method 2 of 2:

Alternative Area Formula

1
Mark the diagonal and side length on your rectangle. If you have a visual in front of you, it will be much easier to see how you’re solving your equation. Draw a simple rectangle with a ruler and create a diagonal line across it to make 2 triangles.^{[6]
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- It’s not mandatory to draw your triangle, but it can help you, especially if you’re just starting out.
2
Enter the length and diagonal into $A=l\times {\sqrt {d^{2}-l^{2}}}$ . Simply plug the length and the diagonal into the formula above. Remember, the diagonal is always longer than the length of the rectangle. ^{[7]
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- For example, if the length is 8 inches and the diagonal is 10 inches, the formula would be $A=8\times {\sqrt {10^{2}-8^{2}}}$ .
3
Solve the equation to find the area. All you have to do now is break down the equation (using PEMDAS) in order to simplify it and get the area of the rectangle. For example:^{[8]
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- $A=8\times {\sqrt {10^{2}-8^{2}}}$ .
- $A=8\times {\sqrt {36}}$ .
- $A=8\times 6$ .
- $A=48$ .

Community Q&A

Question

If the diagonal of a rectangle is10cm then what is its area?

Community Answer

You don't have enough information to solve.
Question

A rectangle with diagonal of 20 cm has sides in the ratio 2:1. How do I find the area of rectangle?

Donagan

Top Answerer

Such a rectangle would have its sides and diagonal in the ratio of 2:1:√5 (per the Pythagorean theorem). The ratio of √5 to the diagonal (20) is equal to the ratio of 2 to the longer side (or the ratio of 1 to the shorter side). Thus √5 : 20 = 2 : x, where x is the longer side. Expressed as fractions, that's √5 / 20 = 2 / x. Cross-multiplying: (√5)(x) = (20)(2). Dividing by √5: x = 40 / √5 = 40 / 2.236 = 17.9 cm. The shorter side is half that, or 8.94 cm.
Question

If the side = 48 meters and the diagonal = 50 meters, how do I find the area of the rectangle?

Donagan

Top Answerer

Let s be the shorter side. Then (by the Pythagorean Theorem) s² + 48² = 50². So s² + 2304 = 2500. Then s² = 196, and s = 14 m. The area is 48 x 14 = 672 square meters.